Board Thread:General Discussion/@comment-173.72.207.167-20130718112956/@comment-6881629-20160225115541

It's mostly random and level based just like loot. For example Balgruuf will give you piece of armor as a reward for Bleak Falls Barrow, as it's usually done at low level you often get steel armor. But if you left the quest until say around level 50 you'd have the chance of it being Daedric instead.