In my opinion, you just haven't been lucky enough to loot a given item, so you don't have the loot unlocked completely.
I will give you an example:
I've killed 6 Yeti, Bunny slipers didn't fall out and the loot is blocked.
The same applies to War Golem, when I was making the bestiary I was not lucky enough to loot Tin Key and I also have loot blocked.
So, in my experience, at least once you have to drop an item to fully unlock the loot.