There is a cell for dead bodies? I thought that was just called delete.
Well. I discovered it in a similar fashion (master trainer for smithing was killed in a dragon attack without me noticing. I worked that out by teleporting to his location after he didn't come to his shop for a week in-game.
I think it's some really strange reason - Essentially what happens is that cell is there so that the actors still exist in the game and can still be referenced, or something. Rather than just deleting them completely. So i think the programming logic is:
If ActorName is Dead
Then do X.
I believe if you just removed ActorName from the save-game completely that the programming logic would get broken.
I.e. If ActorName is Dead
Reference ActorName is invalid.
I believe the reason it doesn't do generic NPCs is because most generic NPCs respawn. Likewise with random NPcs. It's just an issue for named NPCS who are continued to "exist" after their death - for example, if a merchant in a city dies, they have a grave set up for them. The existence of the grave depends on their status as alive or dead, so their body can never be cleared up. Likewise, if their son takes over the shop, the program must still be able to identify the fact that they are dead, rather than just deleting all record.