1099-R Taxable Amount Not Being Recognized as the Taxable Amount

I have a 1099-R where the gross distribution (box 1) shows a certain amount. The taxable amount (box 2a) is a much lesser amount. The distribution code(s) are 7D. No other boxes are checked or have an amount in them. When I enter the gross distribution amount into Turbo Tax it calculates the tax owed. However, when I enter the taxable amount (box 2a) the tax amount owed does not change. So, it appears that the tax software is calculating my taxes based on the gross distribution instead of the taxable amount. Why is the taxable amount not being used to determine my tax liability?  Thanks