The fact that this deduction is listed in section 62(a) of the tax code requires the IRS to treat it the same as any of the above-the-line-deductions on Schedule 1. I overlooked this in my original assessment. I agree that it's inequitable and counterintuitive. I believe that Congress simply made a mistake by putting this deduction in section 62(a) of the tax code and should have made it stand alone so that the deduction presently on line 10b and the Schedule A deduction would produce the same result if other Schedule A deductions equaled the standard deduction. But it is what it is and the IRS and TurboTax must conform.
I suspect that the IRS considered putting it on Schedule 1, but they instead chose to put it directly on Form 1040 near line 12 only because of its dependence on the use of the standard deduction.
As I mentioned, the IRS instructions for the Social Security Benefits Worksheet have very recently been updated to require the line 10b amount to appear on line 6 of the worksheet, so that would be TurboTax's reason for having implemented it this way.
