There are a lot of calculations that go into what portion of the social security benefits that you received are taxable. If any portion of the 1099-R was taxable and was entered as non-taxable it would change that pretty dramatically.
The IRS letter should explain the calculation used to change your return and what else was changed on your tax return. You can check that against your tax return as filed and see if the changes are correct.
@jeannettebiddle3106
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