When did you get the refund? The IRS will send you a letter explaining the change, but the letter usually comes two or three weeks after you get the refund. The simplest thing to do right now is just wait a few weeks until you get the IRS letter.
The IRS calls everything a calculation error. It doesn't mean that the TurboTax calculations were incorrect. It could have been that you entered something incorrectly, answered a question in TurboTax incorrectly, or omitted a deduction or credit that you are entitled to. You'll have a better idea when you get the IRS letter.
