Whoredom as acid smoothie

There's this black dude who works as a bagger at my local supermarket(Vons), and I think he's retarded. He doesn't look retarded, he actually looks well groomed I guess. But he speaks in a really soft, quiet voice, kinda slowly and michael jackson-ish... and moves slowly too, like he's always concentrating. If I was an asshole I'd laugh my ass off any time I saw him. The thing is... I could swear that I once saw him a long time ago talking normally to another employee.

So... Is it a common thing for people in super crappy minimum-wage jobs to act like they're retarded? I guess it may make the work bearable.