I think the balance is knowing what not to commit to the page. The author has to know and understand the motivations of the mystical creatures so there is a coherency to their actions. The reader doesn't need to know every detail. I think some authors feel that if they did the work then they want to put it on the page so everyone can bask in the light of their genius (that's extrapolation based on feeling I don't actually know why some authors decide to write details down).

I feel it's like showing a magic trick and then walking through and explaining how it works. It sucks the "magic" out.