"so much at 10%, so much at 12%, and even a bit at 22%."
That's not how the IRS Tax Table works. None of the seven IRS methods for calculating the tax work that way. It sounds like you found a chart of tax brackets somewhere and did a calculation according to your own concept. If you use the IRS Tax Table, you just look up your taxable income in the table and find the corresponding tax amount in the column for your filing status. There is no math involved when you use the Tax Table.
You didn't say whether the IRS changed anything else in your tax return besides line 16. It's possible that a change somewhere else could leave the AGI and taxable income the same, but require calculating the tax using a different one of the seven methods. A change on another form could change the tax amount on line 16 without changing anything else on Form 1040-SR. The amount of withheld tax does not affect the calculation of the tax on line 16.
If the IRS did not make any other changes, I think there are two other possibilities.
- The IRS could be wrong. It does happen. They might have overlooked something in your tax return that requires using a different method to calculate the tax.
- You could have entered something incorrectly, or answered a question incorrectly, that caused TurboTax to use the wrong one of the seven tax calculation methods. The TurboTax calculation was correct based on your incorrect entry.
It's extremely rare for TurboTax to calculate the tax incorrectly. It would only happen if there is some very unusual combination of circumstances, or if some uncommon obscure point of tax law is involved.
I don't think we can resolve this question here in the Community. There is not nearly enough information to determine which of the seven IRS methods should have been used to calculate your tax, or to calculate the correct amount of tax. Without a lot more information, and since we can't actually look at your tax return, we cannot tell whether TurboTax is wrong or the IRS is wrong.
