Depending on location and what settings the encounter zone has for that location
If you enter that location say at lvl 6, when you come back to it later on at say lvl 16, the level of the npcs / creatures etc are going to be locked at lvl 6 and not adjust to your current level. So thus you have weaker npcs/creatures for the rest of game in that location. Because it does not "reset"..
What Pluto is changing will fix that and allow for certain locations npc/creatures etc to reset in level. And when you come back to location after you have already visisted, things will level up to current lvl..
Hopefully that helps..
Yeah it did help. Well in that case I think it should be neither and be random. If you think about it, from a realistic standpoint. There is this fort next to whiterun called fort greymoore which usually has bandits. So inside of that place there is an old lady that pretty much works for anybody that comes in. So she tells you stories about one night so and so came in etc. The point is that anybody or anything can occupy a deserted area. So you can clear up level 15 out of a dungeon come back and now that same dungeon is occupied with level 30 trolls and vice versa.