You should contact the IRS as suggested in the help article at this link.
Without reviewing your return no one can say why the IRS would use a different calculation than TurboTax does but the IRS will be able to explain their reasoning.
If you agree with them then you will pay the amount due.
If you disagree with them you will have to prove your case with reasoning drawn directly from the tax code. If you think they are wrong and the amounts are significant then it may be a good idea to consult a tax professional in your area.