When you enchant a book with enought levels it can get multiple
enchantments, but they dont fit with each other.
So it can happen that you get a armor enchantment and a bow enchantment on the same book.
I dont think this is intensional because it doesnt make much
sense to have a bow with a "protection"-enchanment.

EDIT: If you combine those books with something it gets all the enchantments from the book, even if
its the wrong tool for the enchantment

What I expected to happen was...:
Get an enchantet book, maybe with multiple enchantments.

What actually happened was...:
Get sometimes a book with stupid enchantment combinations.

Steps to Reproduce:
1. Enchant some books.
2. Look at the enchantments.