Protection and Projectile Protection being applied mutually

So, after AFK fishing for a while, I was looking at the loot and found a Protection and projectile protection book. I though it would not apply to the items mutually, so I was enchanting items via enchanting table and I got a Protection III and Projectile protection IV Diamond Breastplate!

What I expected to happen was...:
They couldn't be applyed to the same item.

What actually happened was...:
They were applyed to the same item...

Steps to Reproduce:
1. Enchant some armor, or fish for a while.
2. ???
3. Thats it.