Hello John,
this is because of the "sero point" where the calculation between the "text paragraph align" settings and the ruler differ. In "left aligned" and "justified" the calculation begins from the left, similar to the ruler, so all works fine. But in "centered" and "right aligned" the calculation begins from the center or from the right, but the ruler calculation is in both cases from left to right.
This is a limitation of the overall system. To get this right, the ruler has to change it's calculation direction by setting "centered" and "right aligned", too. But you continue writing from left to right, so what about the inversion of the "left" and "right aligned" tab stops?
I don't know if you have done this, but you can send a feedback to the developer with some suggestions about this:
http://www.apple.com/feedback/pages.html
