I'm not sure on the question that you specifically asked. I know that our Mini Mill regularly holds +/-0.00025 while circular interpolating a counter bore on one of the parts we make. Generally speaking, if the work is held rigidly, and consistently, the machine will repeat extremely close to what they claim.
In our case, we tend to run small parts, so there is not a lot of weight on the table. From what I understand, though, those tolerances they claim are with an empty table. Someone please correct me if I am wrong in that assumption.