There are a number of things that might impact your benefit this way.
Your 2015 income should not create a large decrease in benefits. It doesn't make sense.
One that might make sense is income tax withholding. Your $2647.50 benefit less 10% might be $2,382.75. If you pay for medicare, that might be an additional reduction.
I don't like letters without explanation, but those two items are my guess. After all, you received a catch up payment to compensate you for your 2015 earnings adjustment.
I would check my last annual statement from SSA, and verify my annual benefit and any subtractions for the prior year.
The only other thing I can think of is an IRS levy, which is available for income taxes in arrears, of 15%.
If neither of these items apply to you, then I am as dumbfounded as your CPA.
If I got it right, though, I would appreciate positive feedback to close out your inquiry. I'mCustomer