Agreed upon? Most of the game is in making up rules and forcing them on others despite their disagreement!

Although that happens with other games also, when there's a disagreement about the rules. It just seems to be a smaller fraction of the game and something that everyone tries to avoid. There are some games that explicitly lampshade this. The official rules of Munchkin say something like (paraphrase) " in a rules dispute whoever shouts loudest is right."