Hearthstone ranked chest rewards

I think the issue come from the level of the opponent you won against. Currently there are not enough players in the game (Which is still in beta) to always find an opponent that allows you to rank up.

As posted by an admin on http://us.battle.net/hearthstone

Matchmaking functions similar to SC2. You have a hidden rating that rises and falls based on your wins and losses, and the amount it rises and falls depends on the rating of the person you are matched with.

As more people get invited to the Beta, you should see an even better spread of people matched against you.

You can end up with any quest if you reroll, so if you have a 40 gold quest and reroll there is a chance to get a 60 or 100 gold quest. The higher value quests are rarer than the 40 gold quests though in my unscientific, personal observation.

This should also work the other way around, though I haven't checked this myself as I generally don't want to waste the 20 gold. So I would not reroll 60 gold quests unless you have no reasonable deck for the heros in question or simply don't enjoy playing them.