Because if the taxable amount in the 1099-R box 2a is the same as box 1, box 2a is added to line 4b and line 4a is left blank, but if box 2a is less than box 2a then box 2a is added to 4b and box 1 is added to 4a.
When there is more than 1 1099-1 box 4b can often be more than box 4a. Box 4a is not used for any calculations, box 4b is what is taxed.
Refer the the convoluted and ambiguous IRS instructions for the 1040 line 4.
https://www.irs.gov/pub/irs-pdf/i1040gi.pdf
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
