Capital gains above 15%

I have inputted all my taxes and TurboTax calculated on the Schedule D tax worksheet that my taxable income excluding capital gains is $400 below the married filing jointly threshold for 20% capital gains tax.  So I naturally assumed I would be paying 15% on all my capital gains.  But within the worksheet it is taking that $400 reduction in my ordinary income below the $553K threshold and allowing that $400 to be taxed at 15%.  And then all of my remaining capital gains after they lower it by $400 is taxed at 20%.  Does that seem to be accurate?  Why would I not pay 15% of all capital gains?