I don't see an error. TurboTax is using a variation of a worksheet found in the 1040 Instructions (page 32) - "Social Security Benefits Worksheet—Lines 6a and 6b".
In the IRS worksheet, line 3 is a collection of taxable items (not including line 2a of the 1040), but line 4 on the worksheet refers to line 2a of the 1040...and as you see, line 5 is the sum of lines 2, 3, and 4.
The TurboTax worksheet saves a step by including 2a in line 3 of the TurboTax Social Security Benefits Worksheet. The result is the same in either case.
Why did the IRS break out line 2a (1040) on to a separate line? I presume because they wanted to separate non-taxable line 2a from all the taxable items.
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