TurboTax does not always simply use the tax tables for the calculation. Some situations require other calculation methods. There are several ways taxes have to be calculated, depending on the situation.
One of the most common situations another method would be used is if you have any Qualified Dividends or Capital Gains. Those are calculated on the Qualified Dividends and Capital Gains Tax Worksheet.
Did you have any Qualified Dividends or Capital Gains? If so, those get a more favorable tax treatment than the tax tables. If so, when you view/print your return with the worksheets, there should be a Qualified Dividends and Capital Gains Tax Worksheet showing the calculation.