I know I guy who's contributed to 50 of the biggest games in the last few years, but he's only been credited 3 times. He works for a top art outsource company.
I recall some companies asking for credits from films. I don't remember what company it was but for them it was a guarantee of you working on that film. As for OP; I've had that happen to me too on a film and some commercials. From what I hear they won't put your name in the credits list if you've been working for the game…
Sometimes even the outsource company isn't credited, hell, he's got work he's done that he can't ever "publish" he can show it in a private setting, but it can't ever end up on the web. SCEA Santa Monica he says was the best studio he's done work for when it came to credit.