I'm going to disagree with the other opinions in that this could certainly be caused by humidity and acclimation. Every wood pro knows that solid wood will absorb moisture and expand. It's rather unlikely that the boards would be milled to different widths and especially that it would vary in width along a particular board. In general, 4/4 lumber is cut to uniform widths then sent through a machine that mills and kerfs the bottom, mills the top flat and cuts the tongue and groove on opposite sides, all in one machine. Another machine generally adds the end matching. Unless something in the milling machine is moving, or the unmilled boards were too narrow, all pieces will be the same width as they exit the molder. Look at this video at about 1:30 to see the molder in action. https://www.youtube.com/watch?v=wc0N6m4RNdY What you're experiencing is probably the effect of moisture soaking in to parts of boards that were more exposed to humid air. Exposed ends will grow more than centers that are less exposed. Look at the your first full row from the top. On the middle board, the end is wider than the one to the left. In the next row down the opposite is true. It's likely that those ends were exposed to more humid air. The problem is exacerbated when the installer doesn't compensate, since the wide ends can kick the next row out of line, leading to more gaps on the edges and ends. The same thing could happen in very dry air with ends shrinking. Exactly how was the wood acclimated? Was it removed from cartons and wrapping? Was it stacked so air could circulate around all sides of every board? What was the temperature and relative humidity in the space during the acclimation period? What is the current indoor temperature and relative humidity? Is the floor nailed down? I had this happen with a very high quality Canadian solid wood floor. Fortunately mine was dark stained Walnut so once the floor was aligned properly and larger gaps were distributed to make two smaller gaps on each edge, the issue didn't show and once the whole floor acclimated in place there was no issue.

@DavidR....the reason why we advise against painting subfloors is it causes issues with permanent flooring choices later on. Example: A glue down ANYTHING (including glued carpet) has a chemical reaction (bond failure) to the paint or the urethane sitting on the subfloor. A tile floor has bonding issues with paint = need a new subfloor surface. Vinyl has chemical reactions to polyurethane/urethane to the point that the two chemicals will stain EACH OTHER (that's right....the urethane will stain the vinyl above and the vinyl will stain the wood underneath the urethane = lose-lose situation). A sealed/painted subfloor (to eliminate pet odour) underneath CARPET (stretched carpet) is a different kettle o'fish. Any other "floor" has issues being put down over top of the painted/urethaned subfloor. I've seen it dozens of times. Homeowners want a quick "pick me up" for their raw subfloors so they stain and urethane them. They live on the stained/finished OSB and then purchase flooring a few years down the line. That's when they find out they need to REMOVE or cover up the finished subfloor. Oops. That's another $2000 renovation cost they did NOT see coming! They find out that gluing down hardwood needs to go down over raw wood, not painted/urethaned wood. Or they find out they cannot tile over the finished subfloor. Or they cannot lay vinyl planks over the urethane/painted finish because the STAINING that WILL OCCUR (no one likes their white vinyl planked floor to turn ORANGE because the urethane below it BLED THROUGH to the surface....that's a REALLY upsetting event). You see where I'm going with this. Anyway....that's more along the lines of "please don't paint the wood" response I gave. There are very few positives to a painted subfloor - your situation is one of the ONLY positives. The rest leave the owner with a bigger mess than if they had just left it alone and thrown some carpet down "in the mean time".

And WHY is this 'nail only'? These are 5" HICKORY planks. A wide plank floor (anything 5" or wider) must have glue assist during install. And hickory has a little bit more movement than oak...which means it needs a bit more stability (ie. glue assist) when installed. The "American Spirit" brand is sold by Floor & Decor. Their installation instructions are...limited. Which means the installer has to have quite a bit of knowledge to know all these things. A guy with a hammer (or nail gun) will have little flooring knowledge. At the price of $5.29/sf (as listed for a store in WA state) the chances of milling issues could be quite high. I think we have several issues going on here. As Oak & Broad suggests, start by CULLING all the boards that are BAD (this is what you are paying your installer to do...but he's not doing it so it falls to you to go for it) and then find out your TOTAL square footage of bad boards. Figure out if they are MORE than 5% (industry standard allows 5% waste for bad wood). If your culled footage is MORE than 5% you have a claim worthy of the paper it is written on. And while you are culling your wood, have someone that is well versed in wide plank install come in and have a look at this install. I'm PRETTY sure you need glue assist on this one. A 5" solid hickory plank (3/4" thick) for LESS than $6/sf is going to need some help. And please tell me that it was acclimated for several days/weeks. And please tell me your installer took moisture readings from random planks equal to 10% of the wood delivered to your home. And please tell me the boxes were opened and stacked PROPERLY while acclimating. And good luck with Floor & Decor. It's my understanding they take quite a bit of time to get moving if there is a claim that they have to deal with. The sooner the better is what I've heard.

Is there any way to measure the temperature at the 'wood-subfloor' interface? Most floor products do NOT allow anything warmer than 85F at the subfloor-floor interface (the temperature found between the two products). The 60 F INSIDE temperature is quite low. It is just on the very edge of "indoor living temperature". Most warranties (paint, carpet, hardwood, baseboards, doors, drywall, cabinets, etc) must have consistent indoor living conditions. The building industry has a definition of indoor living conditions (which are to be maintained throughout the life of the home...regardless if someone is home or not). Relative Humidity: 35% - 65% Temperature: 55F - 85F Your indoor air temperature is only 15 Celsius. That is VERY cool. The lower temperatures have a harder time 'keeping' moisture in the air. That's why you find dew on the grass during the chill of a spring morning. The grass is colder than the air. And the air is already quite chilly. The air has a hard time 'holding on' to the water vapour. The cooler blade of grass is JUST ENOUGH to cause a water droplet to form - pulling the water from the air and onto the earth. As the air warms up, the vapour is HAPPILY stuck in the air. That's why you have hot-sultry areas (like NYC in the summer time) and you have cold DRY areas (like Mongolia in winter). It's how the laws of physics work. I would tip up the heating a snick and OPEN THE WINDOWS to pump up the humidity. I would bet the closer you get to 50% RH and 70 F (20 C) the better your floors will look. Somehow you need to get the humidity and the temperature up AT THE SAME TIME.