The wording of the IRS notice is misleading, which commonly leads to misunderstanding of the problem. When the IRS says that the taxable Social Security was calculated incorrectly, they mean that it has become incorrect because of other changes in your tax return that affect the taxable amount of Social Security. The calculation on the original tax return was correct, based on the income reported on that tax return. But a change in the amount of income, typically because income was omitted or underreported, requires an increase in the taxable amount of Social Security. So the Social Security amount on the original tax return is no longer correct for the corrected amount of income.
One other possibility, as VolvoGirl pointed out, is that you mistakenly answered Yes to the question that asked whether you live in certain foreign countries. If that's what happened the amount on line 6b of your original Form 1040 would be zero. TurboTax calculated the amount correctly based on your answer, but the result was incorrect because your answer to the question was incorrect.
In both of these cases the IRS says that the taxable Social Security was calculated incorrectly, but that does not really explain the problem. In both cases, the calculation in TurboTax is correct for the information that you entered.
