The way I've been thinking about it is this. Among the many reasons people read, there are vindication of expectations and surprise. When you read a strictly formulaic genre story, if you're enjoying it, you're getting the pleasure of having your expectations vindicated in genre-specific pleasing ways. Probably there are surprises in the book too, because nothing good in writing is all one thing. But the balance is the other way. But in formulaic genre there's also the opportunity to turn the expected conventions sideways or upside down and then there's a different pleasure -- not just surprise, also something else I'm not thinking of right now. In genres which are -- I think I'll say "mature" in the sense that a forest can be mature -- you can find almost zen-like shortcuts in some stories, pointing to the expectations the reader has brought to the story Yhe only example I can think of at the moment is the current rash of singularity stories, where almost nobody who writes them now bothers to explain or describe singularity at length. The readers can be expected to bring their own sense of what it is to the story, and even though that sense will vary a certain amount, the story will survive all those variations.
If you ask for a tuna salad sandwich, and when you go to eat it you discover that it has some interesting new pickle in it, and the bread is different from what you're used to, if it's very good, you're less likely to be pissed. Unless you're very unadventurous about sandwiches, of course.