Millimeters not accurate, doesn't go to exact measurements

I have a weird problem, to which I haven't been able to find a solution online due to my lack of Google-fu in this instance

The problem is that when I create a new document, and try to make for instance a box that is 5 mm by 5 mm (click on the artboard, insert the values), I get a box that is 5.292 mm by 4.939 mm. When I click the up- or down-arrows next to the measurements, they instantly go back to the weird measurements. Even when I move the objects around they don't go to the exact measurements, and the Snap to Grid function doesn't work at all.

I've checked all the preferences and document settings, and they are all in millimeters to the best of my knowledge. The artboard is still accurate in it's measurements though. I tried some older files I have, and they seem to work fine. The problem came about some time ago, and I haven't been using Illustrator for a while. I remember doing some work that required I change the measurements to pixels some time ago, but as I said I changed those back to mm.