"Uncredited" actor?!?!

So, I just finished watching Orange County and was rather impressed. It was a quick 82 minutes, but quite funny with some classic Jack Black and a great drug-induced Harold Ramis. It also had cameos from Ben Stiller and Kevin Kline, as a fireman and author, respectively. However, when I was sifting through the credits, I noticed they weren't mentioned. Same thing happened in Anchorman - Stiller, Luke Wilson, and Tim Robbins were all a part of the big "brawl" but weren't in the credits. Ashton Kutcher in Cheaper by the Dozen? No dice either. (Shut up, it had its moments).

Huh? Why? Don't you want the recognition?

I'm not sure how their salaries worked out, but I'm assuming they were paid just like Chevy Chase (credited in Orange County, but in a much smaller part than Mr. Kline). My only thought would be that they thought the film was going to bomb and didn't want it to tarnish their record? Possibly - although Wild Wild West (Kline) and Mystery Men (Stiller) have already accomplished this task.