If you're referring to the 5-year Tax History Report, a footnote on that form says "the tax bracket % is based on taxable income." It's not clear what all you are seeing on that summary form. In any case, here's how you can look to see what method(s) TurboTax is currently using to calculate your tax, which might provide some clue.
Are you using desktop TurboTax? If so, go into Forms Mode. Bring up the Form 1040 Worksheet (may be abbreviated as 1040 Wks.) When the Form 1040 Worksheet is displayed, scroll down to between lines 15 and 16. You should see a Tax Smart Worksheet, which should show you which one(s) TurboTax is using.
If you are using Online TurboTax, you can also see that at any time by going to TAX TOOLS, then TOOLS, In the Tools window choose "View Tax Summary". At the Tax Summary screen go to the left menu column and click on "Preview Form 1040." That will bring up the Form 1040 Worksheet. Scroll down to between Lines 15 and 16 to the Tax Smart Worksheet.
