Mike,
I ran into a problem once where we had excessive cal form error in one position only & it turned out to be that someone had broken off the sphere from the shank, soldered it back on, then filed the excess solder off &, oh by the way,also filed a bit on the sphere - there was one distinct file mark on the sphere that the stylus only hit in one major probe position. Possibly there's a defect or something on the sphere (or I suppose on the stylus??) that only gets approached from the A90,B180 position. I guess I'd watch the touches very closely & maybe try a different cal sphere and/or stylus. Not seeing the error at MEA is really strange though. A definite puzzler...