With ATo I am 74/40 (.16BB/100)
With A9o I am 32/16 (-.03BB/100)

Over about 65K hands, about 600 hands each, all full ring, with about 45K at 1/2. And I think I play A10o way too much. Like jrz said a pair of ten's is more likely to win, you are a little less likely for someone to have a higher ace, and at full ring I think that the str8 does add a lot of value. I think the biggest thing though is that people at micros LOVE to play hands like j10, q10 and k10, suited or not, and you get paid off more when you hit your pair of tens than when you hit a pair of nines.