Tax bracket is only one factor in determining marginal tax rate. Other things such as tax credits and the taxability of Social Security income can be affected by AGI that will also have an impact on marginal tax rate.
TurboTax does not calculate marginal tax rate, it never has. (Even though it might have provided an item labeled "marginal tax rate," it was not.) To determine the marginal tax rate for a particular type of income you must add or subtract that type of income, see the resulting change in tax liability, then divide the change in tax liability by the change you made in income. I commonly use changes in income of $1,000 to make the calculation easy.
Online TurboTax is not particularly suitable for doing this sort of experimentation since you must use a separate online account from the one that you use to file your tax return. I suggest using the CD/download version of TurboTax instead. Not only does the CD/download version allow you to create multiple test tax returns, it also has a What-If Worksheet in forms mode that allows you to compare various income scenarios side by side and to use the following year's tax tables.
