TurboTax uses the mathematical method of depreciation, and does not use the "tables".
Mike outlined the way the mathematical method works. It is somewhat debateable if TurboTax using the correct order of operations, but TurboTax and several other major software companies all use this same method and have done so for many, many years.
If you have the CD/downloaded version, you have the option to the vehicle worksheet, scroll way down and check the box to use the IRS "Tables" for depreciation.
