There are times when you need to use other methods besides the tax table or in conjunction with the tax table to calculate your tax due - one reason being tax rates can be different depending on the source of income.
This is especially true when you have dividends or other capital gains as sources of income. If any of your dividends were qualified dividends, then capital gains tax rates would apply to those dividends. Capital gains tax rates are often lower than regular tax rates, and long term capital gains rates can be as low as 0% for those in the 10 and 15% tax brackets.
In these cases, TurboTax uses the Qualified Dividends and Capital Gains Tax Rate worksheet to calculate your tax due (per the IRS). Since you are showing less tax amount than the tax table (and I see you are using TurboTax Premier which likely means you have investment income), this is probably the cause of the tax liability discrepancy.
If you go to My Account -> Print Center and follow the prompts, then you can print out a copy of your return and find the worksheet used to calculate tax liability if you wish. (You will have to pay for the product before being able to print out any forms).
You can also view a copy of the worksheet above at page 44 of the Form 1040 instructions -
