Get your taxes done using TurboTax

My tax obligation  of $3950 on line 18 indicates a tax rate on my taxable income of 10%. My taxable income includes short term cap gains of $2650. The tax tables say my taxes should have been $4275. If STCG are taxed as ordinary income how does TT determine that my tax obligation is $324 less than what the tables say it should be? What is TT's obligation if the IRS determines that there is an error?