The federal budget is classified into 'on-budget' and 'off-budget' items. 'Off-budget' spending includes the Social Security Trust Funds and the U.S. Postal Office.

The Social Security Trust Funds have produced large surpluses in recent years. Hence, the total budget deficit is smaller than what it would be if we subtracted these surpluses. The Social Security system is funded through dedicated payroll taxes. Thus, using the Social Security surpluses to fund other types of spending, which is essentially what takes place, means that the policy-makers are using one type of tax to fund other types of activities.

Source: Office of Management and Budget, Budget of the U.S. Government, FY2008, Historical Tables [1].