Is the capital gain short-term or long-term? If it's long-term and if your total income, including the capital gain, is low enough, the long-term gain might be taxed at a 0% rate on your federal tax return. Most states do not give special treatment to long-term capital gain.
This is just a guess. It's hard to tell without being able to see your tax return. There could be something else going on. (Don't post your tax return here. This is a public web site.) The income range for the 0% rate on long-term capital gain depends on your filing status, and it's based on your taxable income, not gross income, so it could be affected by adjustments and deductions.
If your tax is calculated on the Qualified Dividends and Capital Gain Tax Worksheet, line 9 of the worksheet shows the amount that is taxed at 0%.
