I think this has to do with the relationship between colour and the reward system in 4E. How the imagined content in the game changes in 4E as the characters gain levels isn't quite the same as it is in 3E. I am not going to pretend to have a good grasp of how this works in either system, but my gut says: in 4E the group defines the colour of their campaign as they play it; in 3E it's established when the campaign begins.
That's kind of confusing... let me see if I can clarify as I work this idea out for myself.
In 3E, climbing a hewn rock wall is DC 25. That doesn't change as the game is played (that is, as fiction is created, the game world is explored, and characters grow). Just because it's DC 120 to balance on a cloud doesn't mean that characters can't attempt it at 1st level; they'll just always fail. The relationship between colour and the reward system doesn't change over time: you know that, if you can score a DC 120 balance check, you can balance on clouds; a +1 to your Balance check brings you that much closer to success.
In 4E, I think the relationship between colour and the reward system changes: you don't know what it will mean, when you first start playing, to make a Hard Level 30 Acrobatics check. Which means that gaining levels doesn't have a defined relationship with what your PC can do in the fiction - just because your Acrobatics check has increased by 1, it doesn't mean you're that much closer to balancing on a cloud. I think the group needs to define that for themselves; as far as I can tell, this is supposed to arise organically through play, and go through major shifts as Paragon Paths and Epic Destinies enter the game.