The wording in the IRS documents about modified adjusted gross income is a bit confusing, but the calculation in TurboTax is correct.
MAGI is almost always more than adjusted gross income, only in a very unusual circumstance could it be less. Use IRS Publication 590-A, Worksheet 2-1 to calculate MAGI from AGI for the purpose of Roth IRA contribution limits if you have any doubts.
To add to the confusion there are separate MAGI calculation worksheets, one for Traditional IRA deductions and another for health care taxes. It takes a bit to work through the fine print and correctly interpret the guidance, but the bottom line is that MAGI will almost always be higher than AGI. Therefore, an IRA deduction will not 'fix' your MAGI to get below a Roth IRA contribution limit.
