Parcan,
Not 100% for sure on being MM's out since if you were indexed one slot out on your chuck jaw slots then you could be out that almost .006". But to be honest, we all have a fixation with 3 jaw lathe chucks and expect them to repeate better than the average chuck is designed to do. To state the obvious, a 3 jaw chuck isn't designed to partially turn work, remove, flip 180 degrees and then re-tighten in the chuck. Yes for lower tolerance work they will do it. But a 3 jaw should be thought of more as a convience than a precision holding method. Ideally you turn the work completely without re-chucking it. If you do have to rotate the work to finish it? Then that independent 4 jaw already mentioned is a far better holding method.
The self closing 3 jaw chuck was invented a long time after the face plate, face plate adjustable dogs, and then the 4 jaw independent chuck. Depending on your work, and your accuracy requirements, you might find a 3 jaw has very little use in your shop. Most of us probably don't use a 3 jaw for or how there designed to be used. You could also have a single piece of swarf inside your chuck that would give you the exact results your saying your chuck has.
Pete