IDK either. I went halfway through a vampire cave and hauled ass out when the master vamp came cause he was too strong but the cave said cleared. Go figure.

I was walking NW of Whiterun when I came across a shack in the mountains. I opened the door and got attacked by the Dunmer Mage/Alchemist resident...don't know who he was. I stole all his stuff and when I left I noticed a "cleared" on the map... So who knows...but I cleared that shack alright.

Had something odd happen shortly after I posted the above. I went and found the orc "town" at the south east of the map, sorta near Riften, the place with the Deadric quest for Malcath. Anyway, you have to escort their leader thru a cave and to a grove on the other side so he can do stuff. When I exited the cave into the grove, both the cave and the grove were marked as cleared. I only killed one group of monsters in the cave, and there were MANY other groups in there, ALL of which were WAY tougher than the group I had to kill, and I hadn't even taken a single step into the grove when it was marked cleared...so yeah...pretty random...