The AoO's give you a simple +4 Ab bonus if I remember right. Or maybe that was just with the rogue feat. Or maybe it was +8 with rogue feat. No idea. But I see that if you have a nice hitting average on a person and you get an AoO you seem to hit so I think it has something to do with your AB. Pretty sure it's +4. Usually happen on sneaks when someone reloads quiver or some spells without like combat casting and all.