I could make guesses, but since you have the desktop version of TurboTax you have everything that you need to answer your own question and answer it exactly.
The United States income tax system is just so complex, so larded up with numerous phase-outs, phase-ins, surtaxes and hidden marginal rates that no "back of the envelope" calculation - and that's what you've got there - can be relied upon to judge the "correctness" or otherwise of a change in tax associated with a change in income.
If I had to make a guess I'd guess that what you are seeing there is some combination of some of the gain being taxed at 20% and maybe also being hit with the the 3.8% surtax on net investment income or maybe the 0.9% Medicare. But that's just a guess and worth exactly what you paid for it. Maybe the extra income put you into the phaseout of the $4,050 personal exemption and the phase-in of the so-called Pease limit on itemized deductions.
Delete the capital gain entry and print out, or save as a PDF, your income tax return "for filing". Put the capital gain entry back in and then print out or save as a PDF your "for filing" income tax return. Put the 1040's side by side and see what numbers changed. "Drill down" into the changes to see exactly what changed, and why. If you complete this exercise then you should understand why you're seeing what you are seeing.
Tom Young
