The tax tables are only one of seven different methods that can be used to calculate the tax, depending on what is in your return. Your return must require one of the other methods. The most common reason that you cannot use the tax tables is that you have capital gains or qualified dividends.
In the IRS instructions for Form 1040, look at the instructions for line 44, starting at the top of the right column on page 41. The seven methods are explained there. If you have a PDF of your tax return with all forms and worksheets, the Tax Smart Worksheet will be in the Smart Worksheets section near the end of the federal return. You can see which method TurboTax used by seeing which one of the 7 check boxes is checked in the Tax Smart Worksheet.
