It's really up to the player to determine how much challenge they want. I'm not only talking about the fact that you can choose different difficulty levels. But the choice of what quest you do and at what time, is up to you.
If you're really looking for a challenge, then do a quest at a lower level than the recommended. If you're in the mood for an easy time, then do the opposite. Overall, as you play the game, you're probably going to end up completing a mixture of both - some quests you complete underleveled, and some you complete overleveled.
So I really don't see the problem here. You can't expect every single quest to be exactly the same level as you are currently. With the amount of variation with how each person plays the game differently, it's impossible. Unless you implement level scaling. But most people hate level scaling.