Here's my theory:
1> Rewards are hard coded and they don't change regardless of when you get them.
2> Rewards are somehow tied into where you are in the critical path progression.
3> Rewards are tied to your character level and increase as you do.
Now I'm sure it isn't Option 3, but I'm not sure about Options 1 or 2...
Here's my example:
**MINOR POTENTIAL SPOILER WARNING**
**LAST CHANCE TO PULL OUT**
**YOU WERE WARNED**
So I've finally reached lvl 40, and have killed more Dragons than I can count. I completed quest to cleanse a Daedra temple and become her champion, given a weapon to use in her name, to bring her glory - awesome graphic, 14 damage sword
(to give you an idea, the Hammer I use is 107 damage). Now to say I was massively underwhelmed by this quest reward is an understatement, and I really want to use it! It looks fantastic, but at almost 10% of the damage of my existing weapon, I can't imagine surviving using this toothpick in place of my trusty, skull breaking, sledgehammer. I'd really been hoping it would have been a two hander too :/