Open the attached PNG image (not required, any image should do I suppose), then go to Image > Resize and just start tabbing through the fields to see the magic happen!
As you can see in the animation above, just tabbing through the fields causes the image's width, height, recommended resize method and projected new size to change (twice!), and Shift+tabbing back causes them to revert to their original values. I suspect it could be due to some sort of internal rounding error, but I could be wrong. Regardless, it's a bug and should be dealt with. Just tabbing and pressing OK without explicitly modifying any values shouldn't cause the image to be resized unnecessarily.
Please also check other dialogs for similar anomalous behaviour.
If it matters, this was observed in 4.1.5 (classic install) on Win10 x64 v1809 (17763.253).