Life Experience
Advancement in D&D can be quick. Very quick. Assuming four encounters in a day of adventuring and twelve encounters each level, this leads to a new level every three days. With little filler travel time a character could conceivably reach 20th level in sixty days. Less if quests awarding xp are also completed, perhaps reducing the duration to as little as 45 days. That’s a month and a half. In the same length of time I worked my first job, I could have hit level cap.
And that is precisely why I have the policy of undershooting CR. Not every encounter, and not by much, but on a fairly regular basis.
And this is why I hardly use XP, and rather go by common sense. Want to be a high level adventurer? It takes years. So yeah, my groups tend to be low level for some in game years.
My solution is to limit characters to one level per year. You still get the XP, but the level gain has to wait. Details at:
http://dnd-wiki.org/wiki/Time_and_Experience_(3.5e_Variant_Rule)