A process of communication is basically one of transformation
This applies not only to fairytales, but all communication in general. When communicating a message, people can try to word it to avoid as much misunderstanding as possible, but it is inevitable that some details may be lost. But communication with the intent to entertain is even more susceptible to changes, because one person may find a certain aspect more entertaining than the other, or find a detail suitable to add, and stories can naturally evolve following what each person finds to be an improvement on the story.