It seems that the Social Security Administration does not just make overpayments to claimants. This is from a statement by the Secretary of the Treasury and the Director of the Office of Management and Budget:
In FY 2006, Social Security Administration (SSA) outlays were $586 billion, $5 billion below the MSR. SSA received a refund from the Internal Revenue Service (IRS) for $5.6 billion as a result of overpayments SSA made for tax years 1999-2005 under the voluntary income tax withholding program for Social Security benefit payments. Payments made to individual beneficiaries were not impacted. This refund reduced outlays and receipts by equal amounts with no effect on the deficit.