Inconsistent Z depth with Increasing Y Positon

Thanks for all the help folks!

So the other day i cut a project and when it was done I noticed that the Z depth was good at y= 0 but got less acurate as y increased. I assume it some out of square or losse something but was looking for a good starting point.

This is the back of my project. There are 3 rectangular shapes being cut all the way through. The bottom right of this image is the orgin 0,0.