Possibly you have foreign earned income exclusion, capital gains, High income not allowing certian losses, other schedule D income, Schedule J or F, qualified dividends, subject to AMT, childs unearned income, which you then do not just use the tax tables. There are so many variables and income subject to different tax rates.
This is a public forum so we can't see your return to explain what you are exactly seeing.
I hope this helped.
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