It's actually all to do with their box office pulling power.
Downey Jr earned a massive amount because he's got the key role in the successful Iron Man franchise.
Angelina Jolie may be a good actress who makes decent films, but she doesn't put bums on cinema seats to the same extent. If I'm wrong about this and she does, then she needs to get herself a new agent.
I am discriminated against because I get paid less for doing a proper job than these entertainment industry folk get for play acting around or singing a song, or kicking a ball about, or telling a joke, or daubing some paint, or showing half an unmade tent in formaldehyde, or just being a celeb, or hiring the folk who managed to get on this gravy train, etc.. Where is my multi million pound mansion ?