One is left to wonder about some backlash creeping in.
Against that, the X & Y axes both show it.
If you start at 0 and make cuts at 20, 40, 60 mm, where is the error?
If all 3 are short by the same amount wrt 0, then I would look at backlash.
If the gap between each (20 mm) step is short by the same amount, I would look at calibration.
Cheers
Roger