TurboTax's Social Security Benefits Worksheet implements the IRS calculation. Compare this worksheet to your own calculation to find why TurboTax calculated a different taxable amount. Given the same inputs to this worksheet and the worksheet that you used to do the calculation, I'm sure that TurboTax would produce the same result, so it's likely that the inputs are different. You'll likely find that the IRS indeed caught some other item of income that was missing from your tax return or disallowed some above-the-line deduction (Schedule 1 Adjustments to income) and the $155 tax difference is due to the increase in AGI caused by the combination of these effects. Without the details needed to perform the calculation, it's not possible to know why the results differ.
Also, the IRS letter should not be construed as indicating that the calculation method of the taxable amount of Social Security income is flawed, only that the taxable amount of Social Security income that the IRS calculated disagrees with what was reported on your tax return. The IRS refers to any disagreement as a "miscalculation." That disagreement is virtually always caused by some other change that the IRS made to your tax return.
