Unbreaking 2 and 1 don`t combine

when i combine the two enchanted books, (levels 1 and 2) in an anvil, they give me a single Unbreaking 2 book. also, when I have an unbreaking 2 item, if i put an unbreaking book on it, i cannot combine the book and item, however, naturally generated unbreaking 3 books exist, i have not enchanted any items and got unbreaking 3