The OMB, on whose data GAO's report is based, tracks not only the amounts improperly paid, but also the efforts made to recover such money. There as well, there seem to be wide differences between government agencies and sometimes between years - some of the statistics look rather dubious in fact, swinging wildly between near-perfect recovery in one year and barely any at all the next, seems hard to believe that's fully accurate.

So you shouldn't conclude that that 2 trillion dollar number is the amount actually lost - it's less as significant amounts of that money were recovered later. But the GAO report, as well as the OMB's PaymentAccuracy.Gov website that it's based on, are frustratingly vague on what seem like very basic questions, such as indeed how much the actual net amount lost is.