The Romneys’ generous charitable donations in 2011 would have significantly reduced their tax obligation for the year. 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% in income taxes in each of the last 10 years.So Romney voluntarily forked over more money to the federal government than he needed to because he didn't want the percentage to look too bad.
ADDED: Sorry for the typo in the headline. It's 30%, not 40%. He's not that generous.