When you enter one taxable transaction, you can't just watch the monitor. You increased your overall adjusted gross income and with that come many other changes in your return, not just the incremental tax on the one transaction.
Even though the Capital Gains is taxed at a different rate it still increases your income and AGI so you might be losing some of your credits or deductions, like the EIC or the limit on Medical deductions (only over 10% of your AGI) or Employee Job Expenses (only over 2% of AGI).
For the Desktop version you can switch to Forms Mode and open the worksheet to see it. Click Forms in the upper right (upper left for Mac) and look through the list and open the Qualified Dividends and Capital Gain Tax Worksheet.
