Did you enter any Qualified Dividends or Capital Gains on your tax return? If so, the taxes on your taxable income are not calculated using the IRS tax tables or the tax computation worksheet. The taxes are calculated using the Qualified Dividends and Capital Gains Tax Worksheet.
Go to this IRS Form 1040 Instructions page 36 for the worksheet - https://www.irs.gov/pub/irs-pdf/i1040gi.pdf#page=36
