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"Is this related to humidity levels inside the house?"

No. Millworkman sums it up quickly.

"another picture. Is this something that can be fixed with wood filler?"

More like a few tubes of caulk. Too bad you're dealing with this. It almost looks like someone is trying to install this as a floating floor? I see no nails or staples. Or is it just laid loose for now?

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.

Cheepchinesecrapola.

Although this can be RH related, most likely and more common millwork for prefinished products. Floorboards can be sorted out before fastening. Just extra labor you need to pay for a tighter fit.

The acclimation process is never precise. The way the planks are racked and stacked during acclimation is intensely important (photos of the racking process would be helpful). They "living conditions" that were tightly controlled DURING the acclimation process will be intensely important (documentation of the indoor conditions will be needed). The HUMIDITY control is of HUGE importance and must be WELL documented (low humidity is JUST AS BAD as high humidity).

And then the Moisture Content readings (MC readings) of the wood when delivered, during the acclimation process and then AT THE TIME OF INSTALL will be HUGELY important (documents should be available for your review if they were done).

The MC of the subfloor will be of import. This is especially important on the day of install. A wooden subfloor will have to be within 3% of the wood planks. The TYPE of wooden subfloor will ALSO play a role. Plywood is the 'best' option, OSB can be one of the lower level options...but there are some brands that conform to Plywood levels....you will need to know what is sitting under the wood planks and how long it has been acclimated. A concrete slab will need to be within a certain range (depending on the manufacturer's requirements).

Once the "complete" picture has been put together, then we can start to assess whether or not it is the material (very rare) or the home setting (quite common) or the installer (quite common).

In essence, we need more information. See how much you can get out of the installer before pointing at the material.

It's Floor and Decor. Enough said. Take it all back. Go to an good old fashioned flooring store that has been in business for 10 years or longer. Take their advice. Buy their product. And use their installers.

I used 5" hardwood planks for my floors. We have a few areas in the entire house where there was a small gap or a small chip off of the board that ended up being filled with filler. You can see the filler. It's no big deal when it's just a few places in an entire house. But, your floor...if you try to use filler to fix that, I think is going to look awful. Don't do it. Your floor needs to be a lot tighter than this.

What is your flooring person using under the planks? Is that red rosin paper?

I don't really know what's causing your floor to be like that, but I just wanted to throw out what I think could be another possibility (or contributing factor). Could it be, at least partially, from the installation? Is your flooring person really working to get your floor tight? It's not an easy job and it's a bunch of hard work. Did he start off with a row that's not straight or something? How much experience does he have laying down 5" planks? They aren't easy to lay down and get tight and, from what I experienced with my floor, a small error in laying them down (or from using a bad board) can perpetuate itself, with row after row being affected.

I don't know if it's your wood, but, it's true that you really do need to start with a quality product and acclimate it correctly or it'll never be right.

It looks more like badly milled material, especially if this also at the ends of the boards. also is your contracting gluing and nailing or just nailing? It is recommended to glue and nail anything wider than 4".

You might take some very careful/detailed measurements of the boards to see exactly what's going on with the width. In other words, I'd check to see if the boards are uniform, or not, in width.

Looks like the installation is a glued floating floor over felt. Are the flooring engineered?