Cheaper naming when putting enchanted books together

I was testing about some enchanted books while finding this bug.
The bug itself is kinda simple to tell i guess. When you put in 2 books of (when i found the problem Protection III to combine them together and rename it, it costed me 12 levels. Then i wanted to take one of the books away to see how much it costed for only renaming one book, and that was 13 levels. So in fact, combining 2 Protection III books to a protection IV book and renaming them costs one level less, than renaming one Protection III book. In fact you're just getting a better book with renaming!

That can't be on purpose. Tell me, if i should try and do a video about it, if this was a bad explaining !