When any Enchanted Book is obtained with a level higher than is possible by default–either through a loot table or by using the /give command–it can only apply the vanilla maximum level when used in an anvil. For example, a Diamond Sword combined with a Sharpness VI Enchanted Book will only result in a Sharpness V Diamond Sword.

How to Replicate:

1. Obtain any Enchanted Book with an enchantment level higher than its default limit
2. Ex: /give @p minecraft:enchanted_book{StoredEnchantments:[ {id:"minecraft:unbreaking",lvl:4}

   ]}

1. Attempt to combine the book with an item in an anvil
2. The item will only be enchanted to its default limit (Unbreaking III if using the above example.)