There are several possibilities. See the TurboTax FAQ below for some possible reasons.
https://ttlc.intuit.com/replies/3288510
If you still have questions, please contact the IRS for clarification. The phone # is 800-429-1040.
*** I am NOT a tax expert. I am a seasoned TurboTax user, and volunteer to provide assistance to TT users. Nothing I post is to be considered TAX ADVICE; I bear no legal liability for responses.***
