If your opponent has a better A, you have 12 outs, which will earn you the pot almost half the time. If there's a card on board that is bigger than her kicker (ie she holds AJ and the board is AQ2, you have an additional 3 outs to a split if another 2 falls). Your hand is even better if the pair you have isn't top pair, as you will have an extra 2 outs to your trips, which gives you an edge vs top pair.

If you "know" your opponent has a bigger kicker, then calling on the river will often be wrong. But sometimes you will be wrong also. It depends how certain you are that your opponents kicker is better than yours (and this also depends on the size of your own kicker). This will also depend on how high the top pair is.