If you had to set your steps very far from 10160, I'd say something is loose and moving around some. Spanner nut pre-loading bearings? A block not fully tight?
When I got my kit, I put in 10160, and every move in the same direction checked dead on and when I changed direction there was .0007" backlash and every move in that direction was dead on.
After saying all of that,.......... Yea, I don't know either!
sorry
Good luck,
Jack
P.S. If you had dirt or something in the collet, the endmill can get out-of-round. You can check that by putting an indicator on one of the flutes of the endmill and spin the spindle.