I know there are rules or laws with which I'm not familiar with that dictate when people get credit. And I suppose some frozen corpses and person in a winter outfit shooting at a dog might not constitute a "based on characters created by" credit. I can't recall if Aliens credited H.R. Giger or if he was involved in any way beyond the first film. I know I've seen a "based on a screenplay by" credit before. I'm just curious what makes those kinds of credits pop up sometimes and not here (again, unless I missed something).