Mitt Romney paid extra taxes because he was in the "unique position" of having publicly claimed a 13 percent rate, the Romney campaign said Friday.

Romney and his wife, Ann, paid more taxes than they had to in 2011, choosing not to claim deductions for $1.8 million in charitable donations. The Romneys donated just over $4 million to charity in 2011 but claimed deductions on only $2.25 million.

In August, Romney said he has paid at least a 13 percent effective rate in past years, and claiming full deductions would have lowered his rate below that mark. The donations would have brought Romney down to a 10.38 percent effective rate, according to CPA Steve Frushtick.

"The Romneys thus limited their deduction of charitable contributions to conform to the Governor's statement in August, based upon the January estimate of income, that he paid at least 13 percent in income taxes in each of the last 10 years," Romney's blind trustee, Brad Malt, explained in a blog post Friday morning.

"I did go back and look at my taxes, and over the past 10 years, I never paid less than 13 percent," Romney said in August, following up on a question asked weeks earlier by ABC's David Muir. Senate Majority Leader Harry Reid had claimed that Romney paid no taxes in at least one year, according to a source Reid would not name.

Romney aide Michele Davis, a former Treasury official hired by the campaign in August, elaborated Friday on why Romney chose to pay more than he owed.

"He has been clear that no American need pay more than he or she owes under the law. At the same time, he was in the unique position of having made a commitment to the public that his tax rate would be above 13 percent," Davis said, in response ABC's email request for more information about Romney's decision. "He directed his preparers to ensure that he is consistent with that statement."

Davis responded in light of another Romney statement. In July, Romney told Muir that if he paid more taxes than necessary, he wouldn't be qualified to be president.

"I don't pay more than are legally due, and frankly if I had paid more than are legally due I don't think I'd be qualified to become president," Romney told Muir. "I'd think people would want me to follow the law and pay only what the tax code requires."