You compute your income tax based on TI (taxable income), not AGI (adj. gross income).
AGI less itemized ded/or std ded, and personal exemptions = TI
Then you add or subtract many things (AMT, credits, SE tax, etc).
Romney's tax is 14% because it is based primarily on qualified dividends and long-term capital gains, which are given a preference at 15% due to the Bush tax cuts passed in 2001. ("Carried interest" is huge loophole that should be closed that gets treated like LTCG's).
How many voters realize that Obama extended the Bush tax cuts for 2 additional years on 12/17/2010. The cuts were set to expire, but he'd rather complain about them. Romney has not done anything wrong in that connection.