Per the ARS/USDA site regarding the updated 2023 map:

"Plant hardiness zone designations represent what’s known as the “average annual extreme minimum temperature” at a given location during a particular time period (30 years, in this instance). Put another way, the designations do not reflect the coldest it has ever been or ever will be at a specific location, but simply the average lowest winter temperature for the location over a specified time. Low temperature during the winter is a crucial factor in the survival of plants at specific locations."

My initial comment about playing it safe is further reinforced by this info. I knew it was about average minimum temperatures, not necessarily the coldest it has ever gotten in a location, but I think that bumping almost everyone up half a zone or more knowing that it can get colder in their location (and has done so very recently) may be a bit irresponsible. And I don't know how they can use the word "thrive" the way they do (used elsewhere on their site, as Dan's comment shows).

If someone plants a flower, herb, shrub, berry, fruit, or tree that is supposed to to be hardy in zone 6b and it gets colder the following winter they just lost their plant(s). Not a huge loss since it was just a single growing season. But if it happens 5 years down the road then that is 5 years of growth for a woody plant that has just been lost. If that was a fruiting plant it may have just started fruiting or was about to. If it was a shade tree that could be 5 feet of growth or more, depending on species.

Rodney