OPM 1099R

CSRS retiree, OPM 1099R has box 1, gross distributions, box 2a, taxable amount and box 5, employee contributions/ROTH/Insurance premiums. Box 7 has a 7, box 9b has my total contribution (which I use to fill out a simplified method worksheet (SMwksht). On my 1099R, box 1 minus box 5 never equals box 2a, not even close. When I have completed the SMwksht it lowers the box 2a amount by a miniscule amount (about a third of the reduction the SMwksht calculates. So box 1 minus box 5 is $5700 less than box 2a and the SMwksht amount of approx. $6800 only lowers box 2a by $1600. Why put $5700 more than can be accounted for, in box 5? It feels like I'm leaving too much on the table. Can someone please make sense of this. Thank DJ

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