I believe @Christing123 is correct, some deduction or credit is changing when you add more income, so your taxable income is going up more than the amount of the additional income that you entered.
You can look at your form 1040 while working online with TurboTax to see what is changing as follows:
- Tools
- Tax Tools (left side of the menu bar)
- View Tax Summary
- Preview my 1040 (left menu bar)
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