I assume you mean that you received a refund other than the refund shown on your tax return. If the additional refund is a small amount it might be interest that the IRS pays if they take too long to send the refund from your tax return. Otherwise, in a couple of weeks you should get a letter from the IRS explaining what the additional refund is for.
TurboTax calculations are usually correct, but they are based on what you entered. If you made a mistake in an entry, or answered a question incorrectly, it's possible that the IRS would catch the error and correct it. Wait and see what the IRS letter says.
