Yes the taxable amount on the "b" line can be more than the "a" line. If the taxable amount is the same as the total distribution then nothing shows up on a. The amount would only show on b. If you have more than 1 item then it can be different.
I just checked in my 2021 program. When the gross amount is the same as the taxable amount both lines 4a & 5a are blank and the taxable amount on line b.
