Sometimes that happens. You used "do-it-yourself" tax software. All of the calculations were based on the information that you entered. Did you leave out any income? Enter something incorrectly? The IRS can catch mistakes and change your refund. What exactly does your letter from the IRS say? TurboTax does not get any information from the IRS after you file your tax return, so no one at TT knows what the IRS is changing on your return or why they are making a change.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**