I used TurboTax for the return. On form 1040, the tax in line 16 is so much higher than the calculation of taxable income on line 15. I wonder why it's different.

you mean line 16 is higher than what you get if you calculate the tax yourself?

line 16 tax is not calculated directly by putting line 15 into tax tables, there are other intermediate schedules that may apply for your situation, and capital gains/qualified dividends are taxed separately.  In the PDF with all forms and worksheets (or in Forms mode on TT desktop) you should have any additional forms related to the tax calculation and in particular look for the "Qualified Dividends and Capital Gain Tax Worksheet" which does that calculation and determines the amount of taxable income to send thru the tax tables.