“Anyone know what this is for?”
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I think that the Social Security Administration recomputes the monthly benefit for folks who have had an additional year of earnings that wasn’t factored into the original computation & that can result in a somewhat higher monthly benefit ongoing. Plus a separate “retroactive” payment for the amount of monthly increase times several months (back to January?). The “bonus” may that retroactive payment.
Or maybe it’s a bribe. /sarc - just kidding
I think you are correct.