alt Hacker NewsThat’s not a question about the underlying rules of a fictional work but your perception of how they are created. It’s possible to have a completely well defined fantasy setting with exact rules without the reader being aware of what those rules are or even knowing it’s using well defined rules.
Consider The Martian, early versions where posted online and the author changed what resources the character had to work with at the beginning. So what feels like a creative solution to limited resources was really giving the character exactly what they needed after a solution was found. Only examining a work we can’t distinguish ‘soft’ physics updated as the plot demands from a story based around fixed rules.
You seem to confuse the creative process with the final product. The rules can change during the creative process. It's the final product that I judge as a reader - I won't bother going over the inconsistencies in Harry Potter here, it's been done ad nauseam elsewhere. The physics doesn't change over the course of the story of the Martian.