I've adjusted the resolution, it appears spot on. When routing out 4" test squares, squares close to x0 will be nearly spot on in size, but squares close to xmax exceed the dimension by 20 mil. What can cause this?

I've removed the rack for the rack and pinion and its curved about 1/8". The holes for the rack are also following this curve in the gantry. I don't think this is the issue but it might be part of it?