There could be other credits on your tax return such as education, child care, or child tax credits that may be unfavorably impacted by adding additional income. Also, alternative minimum taxes, net investment taxes and other taxes are also sensitive to income adjustments.
TurboTax uses tax tables to calculate your tax, as opposed to using the tax rate schedules, so there may be a slight difference between your the taxes calculated using one method or the other.
To nail it down, you would have to print your return before the additional income was added and after to see what changed, then investigate as required.
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