While it seems counter-intuitive, there are a number of things that can happen to cause this that are correct. Sometimes the entire situation will correct itself when the balance of the data is entered. This can happen if a reduction in income reduces credits that are income-based (such as child tax credits or earned income credits) or if the additional deduction causes the taxpayer to be subject to AMT.
It's hard to know without information on other aspects of your tax situation, but this is not an unheard of phenomenon. The best way to get figure out what's causing this is to preview your 1040 form before and after the entry.
If you are using a desktop version of TurboTax simply click on the Forms icon in the upper right of the screen (upper left for Mac). If you are using the online version follow these steps:
- Click on Tools in the left navigation panel
- Then click Tax Tools
- Click on View Tax Summary in the pop-up box
- Click on Preview my 1040 in the left menu panel
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