Well, I think it's pretty clear from the rules that it's not passive learning. It's becoming a mighty hero, a paragon of heroism, and epic being!
Gygax expressed it nicely in his DMG (pp 81, 111-12):
A character under magical attack is in a stress situation, and his or her own will force reacts instinctively to protect the character by slightly altering the effects of the magical assault. This protection takes a slightly different form for each class of character. Magic-users understand spells, even on an unconscious level, and are able to slightly tamper with one so as to render it ineffective. Fighters withstand them through sheer defiance, while clerics create a small island of faith. Thieves find they are able to avoid a spell's full effects by quickness . . .
[T]he accumulation of hit points and the ever-greater abilities and better saving throws of characters represents the aid supplied by supernatural forces.
The abilities of 4e PCs don't represent their passive learning: they represent their will (as manifested in a fashion appropriate to their power source and class) and their connections to supernatural forces, which permeate the world of 4e D&D.