I think it would be because now that he knows you'll be making a move X% of the time and actually have a hand that beats him X% of the time (since he knows you have two red Aces), but whether he calls or folds doesn't change your EV from the hand and he can do nothing better than guess if you're ahead of him.

I'm not sure on how to do the math on this (even though I understand it as I read in TOP) but I believe the 15% is just the number thats needed so that he can bluff in such a way that makes what I stated above true (that mathmatically the SB can not change the BB's EV on the hand without a total guess)