Hi George - it is unlikely to be an issue with the CAD/CAM software itself. Maybe post processing, but accuracy issues like this are usually due to the machine itself or the control software. I would ensure you machine is properly calibrated but testing it moves the correct distance when commanded. Do this over various distances as any error in the steppers is usually more pronounced the further the machine travels. I made a post on how to correct the error if you have any. Hope this helps.
-Tom