To the best of my knowledge - which is limited - this kind of billing is not a matter of SAG rules but is negotiated by the actors.
If I'm an actor who has been successfully negotiating a certain level of credit and pay in movies and I accept less than that for a given appearance when I could get more, I'm taking money out of the pockets of people like my manager and/or agent as well as giving leverage to the producers on the next project ("You worked for less for Codswallop: The Movie
, didn't you? Why them and not us?"). If I'm an actor who's in demand you're not only buying my talent but my reputation. Whether Marlon Brando delivered a performance worth what he was paid for Superman
, for example, is a matter of opinion; however, Richard Donner noted that signing Brando essentially got the movie made and got it the budget it needed.