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.
Like for example it increased your AGI and that would decrease some deductions if you itemized on Schedule A. And by increasing your AGI it might reduce some credits you were getting like EIC. And can make more of any Social Security taxable.
You would have to compare both returns line by line before and after you entered the income to see all the changes.
It may have entered it as self employment income and you pay self employment tax on it. You will pay Self Employment tax (Scheduled SE) on a Net Profit of $400 or more on Schedule C in addition to regular income tax on it. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
