Why are long term capital gains rate 19%, but my income is in the 15% bracket. Test...modified my LT gains then divided the difference-in-tax by the difference-in-gains.

Our married, joint income was less than $400,000 which put us in the 15% bracket for long term capital gains. However when I tested modifying my LT capital gains by several thousand and calculating the rate, by dividing the difference in tax by the difference in gains, the rate came out to nearly 19%.  Please explain.