I have looked into this quickly just now, and I can tell that if you make a cross inside the circle instead it pinpoints the centre prefectly. If you make a "plus" is it out by a small % EVERY time no matter how big or small the circle is.

I would hazard a guess and say that this is due to something very basic we are missing OR something very complex about pixel density and the fact that a vertical line has to consist of pixels directly above/below each other whereas a diagonal line "skips" pixles givin the illusion of being accurate.

Both of my methods pinpoint the same place mathematically, however they are definitely rendered differently.

Can you chance to a X in your circle without major problems?

Regards
Paul Lewis