^not to make any judgments or presumptions, but often when people can act sober on large amounts of any substances, its indicative of a problem.
Since you're in the US its safe to assume the legal BAC in your area is .08 and at some point was .1 or even higher. They determined the old .1 BAC legal limit by testing people's driving performance and averaging. Later on they realized that alcoholics naturally had a higher tolerance and could function better, so while .1+ was safe for alcoholics, it could be quite dangerous for most people so they lowered the legal limit nationwide.
Its roughly the same for any drug... people predisposed to addiction issues often can 'handle' themselves better and their use is less apparent to people around them. This is one way that a lot of peoples' habits get so bad because the people in their lives who may intervene earlier don't see the extent of the problem.
Now whenever I hear people claim that they drive great drunk I think odds are that they're either a drunk (or on their way to becoming one) or their delusional. Either way, not great for them.