Capital Gains on Turbotax Premier Calculating Incorrectly?

I have a question on how the software calculates capital gains. I had a refund of $5,519 showing on the software. Then I entered my long term capital gains of appx $23,000 and it decreased my refund to $3,625. I should have been taxed at 0% because our income was not above a certain threshold. I went to the capital gains calculator and it does state we were taxed at 0% for the long term gains in the software. Why would it decrease our refund if we were taxed at 0%?