There could be many reasons for invalid W-2s, but the main reason is most likely fraudulent SSNs. In fact, this is one of the arguments used by the pro-illegal groups: that illegals are paying into the system but will never see any return.
Of course, they fail to mention that illegals claim 99 dependents, and therefore have -0- income tax withheld, but they are correct that SS payments cannot be retrieved. Yet, there is a plausible expectation that if amnesty was granted, there'd be a huge class-action suit filed to have some guestimated amount granted to them.
As to the total number of fraudulent docs floating around, well thats in the billions & trillions. It's actually a pretty big business, one that is controlled by some Mexican crime families.
Thanks