I think that people who go into a fantasy game should go into it with the assumption that they, the players, cannot understand all of the rules of the world.
i think this is a healthy approach to fantasy.
Where there is similarity to Earth, it is a matter of coincidence, and/or convenience. Not a matter of "reasonable expectation".
Now the trick is that, whenever the DM chooses to use elements where something common to Earth behaves differently in the fantasy, it is on them to provide tools to the players to allow the PCs to react appropriately based on the PCs knowledge of the world.
Edit: note that game mechanics for PCs are there, specifically to do this last job.