The two things are completely separate, but having a fixed expression is a necessary requirement. If I go to a sporting event and describe it in an article, then my article is copyrighted, and derivations of it are protected (translations etc.). But there could also be other people who wrote different original articles describing the same event, each of them is copyrighted, but none a derivation of the other or of mine. But the fact that all of those articles are different descriptions of the same match doesn't mean they're not all copyrighted, and that their derivations aren't protected. Each of those articles is a fixed expression, but the game itself isn't.