What?

It is possible that objects are printed distorted depending on the position on your print bed.

Or distorted everywhere.

Reasons non linear

This can be caused by

• Broken belt core and stretched caused by over-tensioned belts. ⇒ new belts
• High forces causing temporal teeth deformation ↦ reduce print speed (try 20mm/s)/acceleration/jerk (try 5) 
• Non centered pullies causing alternating loads/radii ↦ new pulleys or center if possible (rotate)
• Dirt (or waste filaments) in the belt or pulley ↦ clean belts and pulleys 
• Alignement of the Belt is not with the Pulley and causing deviation angles ↦ align belt path with pulley surface (stepper axis)

Reasons for general distortions

• Deformed teeth due to over tensioned belts ↦ loosen your belt
• Slack due to under tensioned belts (backlash test) ↦ tension your belt 
• to much clearance in the mechanics ↦ tighten mechanics
• to much friction in the mechanics causing higher belt loads ↦ calibrate bearings
• non orthogonal gantry (print ultimate cube to compare diagonals) ↦ tramming axis

How to check?

You simply print the model twice .. and hold together - one turned 180°.  Repeat for the other axis 90° rotated  ( diagonal for core XY printer)