I believe this is due to how Photoshop handles calculated scaling.
If you multiply your two sizes, you end up with 5551mm². I believe this is a size Photoshop can't really display in pixels. What I mean with this is, Photoshop can only assume how big that would be in pixels. If you're zooming to a picture for example, your picture might look blurred if you zoom at the factor 1.5x .
Photoshop has to guess using calculation what pixel belongs where when scaled out. I would call this the "half pixel problem", because this is what this is all about pretty much. You can't display half a pixel. You simply can't.
tl;dr What Photoshop does here, is simple. It looks for the next size that it can diplay without problems.
At the same time, it has to use a size you understand. So the trade off here is, Photoshop might round up or round down your numbers a bit, but as a reward you get a guarantee Photoshop can properly handle that size. Because this is at document creation level, Photoshop can't just assume like it does when you're zooming. It has to write something static to your disc so your OS doesn't get confused.
Different story with vectors, those are entirely calculated.