Capital gains being taxed despite taxable income below $83,350

My taxable income is well below the married filing jointly threshold of $83,350, which as I understand is the point at which capital gains are taxed at 15%.  But it appears in TurboTax that my capita gains are being taxed at 10%!.  I tested by changing the amount of capital gains, and the federal refund amount changed by 10% of the change I entered.  What am I missing?