TurboTax correctly calculates the amount that should be on line 6b based on the amount of your Social Security income and other income. If the amount on line 6a is correct but the IRS disagrees with the amount on line 6b, it means that the IRS disagrees with the amount of some other item of income that you reported on your tax return, perhaps not recognizing the use of Worksheet 1-1 from IRS Pub 590-B for determining the taxable amount of your traditional IRA distribution. The effect on line 6b is only a side-effect. The IRS refers to any difference like this as a "calculation error." It doesn't mean that the line-6b disagreement was the result of the line-6b calculation itself being done improperly by TurboTax.
If the IRS legitimately disallowed the use of Worksheet 1-1 from IRS Pub 590-B and TurboTax used this worksheet, you would have a reasonable claim under TurboTax's accuracy guarantee. TurboTax is rather liberal in the use of this worksheet, in my opinion.
