Allegory, symbolism, critical analysis... sometimes we just overdo it. Books have been used to propel ideas through subtle references and metaphor, and often the reader easily gets the point: Narnia is about Christianity for instance, but no one would really describe this as subtle.
Sometimes a classic book becomes divorced from its historical context over the years. Or the intent of the author is ignored in favour of some other unintentional message, or sometimes the public as a whole just doesn't really 'do' nuance. And of course occasionally the movie is just a bad adaptation.
It raises an interesting question of who decides what the meaning of a work truly is: is it the person who planned and executed the storytelling or is it the person who in the end will pay for and consume the work? It can be argued that as the consumer and members of the court of public opinion that we decide what the true meaning is. But when a person takes the time to write the damn thing we should at least assume they get a pretty big say in the matter.
But sometimes a work is created to carry a specific meaning and instead of attributing some outside truth, we instead suggest it's about ghosts or some other nonsense.