Velvetychocolate (great username!), I think there might be vanity sizing in footwear. That's why I cling to Euro 39.5 as my touchstone. Although occasionally a Euro size is way off, I find the ones I come across are fairly consistent. On the other hand, I own US (and Australian - despite what the conversion charts say, Australian and US sizes are normally the same) shoes that are all over the place.
I have shoes sized 8.5, 8, 9 and even 7.5? What's with that? I do have to size down to a 39 due to lack of half-sizes most of the time (my right foot is 39, left 39.5) but that doesn't account for the wide variation.
It might help to know that Euro sizes don't have exact American equivalents, as the size increments are different. Euro increments for full sizes are 0.6667cm; US increments are 0.8467cm.
This might be why some reviews by Americans of shoes that have had their Euro size pre-converted into US size by a website, say that shoes run way too big or small (I know shoes can run untrue to size, but my Coclicos are the truest to size I've had for years, even though websites said they ran small).
ETA this is also why Coclicos are treasures: they do come in the relatively rare Euro half-size, which is one reason why the fit is virtually custom.