Tax rate on 2nd home sale

We sold a 2nd home in 2025, make less than $600,050 married filing jointly, including the proceeds from the sale, so I expected and planned on paying 15% tax on the gain, but the program is computing a 20% tax on the gain.  Just want to make sure the program is calculating it correctly?  I am sure it is but why 20% when from what I read it should be 15%? Thank you!