Vincent, FWIW I have not been seeing that. And up until you said that, I had been considering both the WWI and WWII draft registration cards as great sources. I don't think they really constitute "proof", but they were completed by the subject himself, who knows all that personal data better than anyone, including how to spell his own name! And they even include a physical description.
Just curious if you're seeing a really high percentage of inconsistencies and what other sources you're comparing? I guess I would be inclined to trust actual birth or death records or a tombstone inscription more than a registration card, but not census data or marriage records.