You appear to have used the tax table to calculate the tax. The tax table is only one of seven different methods that can be used to calculate your tax, depending on what is in your tax return. Your tax return must require one of the other methods. The most common reason that you cannot use the tax table is that your income includes qualified dividends or long-term capital gains, which are taxed at lower rates.
In the IRS Instructions for Form 1040, look at the instructions for line 16. Starting at the bottom of the right column on page 32 it explains the seven methods that can be used to calculate the tax on line 16.
