Because you say "appears" too high, I suspect you're just misinterpreting the bottom line. The capital gains tax is calculated on The Qualified Dividends and Capital gains worksheet (for some people it will be the Schedule D worksheet). Review the worksheet for the calcs to see if you are really being taxed at 20% instead of 15%.
The net investment income tax (the 3.8%) is calculated separately on form 8960.
