I think the previous posts are right about difficulty level, but I suspect you get better random loot based on character level - I definately felt I got more epic stuff at higher levels, even when I went back to unlock old chests with my newfound lockpicking skills.
I could be wrong though...
This is a good point. I wonder if the loot is random then? If you open up earlier chests with a much higher level character later on, have the developers ensured that the game gives you better equipment?
