We can't see your tax return so we don't know what might have happened. You can try calling customer support and asking them to do a screen share with your account.
Turbotax guarantees accurate calculations, but doesn't guarantee that your inputs are accurate, and there is no way of knowing what happened without a detailed look at your return and the IRS transcript. Also, the accuracy guarantee will pay interest and penalties if you owe them as a result of a calculation error, but there is no payoff from making a claim on the guarantee if you owed less tax.
