If Slipslix acknowledges we've been to Dun Scath that means we've already beaten scathatch and Diablos but when we return there and see Proto Ultima Weapon it's because he's detected either of the two void sent monsters presumably, when we're there in Dun Scath though it's like we are there for the first time but the quest dialogue implies we've already been there.

Really gets that noggin joggin.
What's the deep lore behind this?