@WhammeWhamme maybe I’m using the wrong word so hopefully you can help me better explain what I mean, but here’s what I’m thinking at least.
Believable comes with consistency (But again maybe not right word, and maybe not even verisimilitude). However, if the fluff describes one situation and incident, and that incident implies all sorts of rules of physics for that world, then a very similar situation occurs, and it progresses completely differently from that other situation, and the results are also totally different... that breaks someone’s belief in that other situation. And the whole world just becomes a joke.
Instead you have a story with a lot of different incidents that don’t all go together, and conflict, and may as well be coming from different stories where each has its own world and reality with different physics but don’t tie in together.
"Internal Consistency", yeah.
Note that this has nothing to do with historicity.