I could be wrong, but I'm pretty certain he paid more than he could have for appearances sake. Had he taken his maximum deductions I've read that he could have paid as little as 9%.
How would that look to the general public he's trying to get to vote for him? More, with the Dems pounding on the fact that he wants to produce more tax cuts for his tax bracket... how would that effect the perception of fairness that the independents and conservative Dems he needs to win?
He decided people could deal with that number because that is the number they had already digested. If the reality was actually much lower, people would have looked askance.
Is that fair or right, probably not. As Romney correctly said most of us try to pay as little in tax as possible. The fact that he felt the need to somewhat artificially inflate his number is actually significant.