It makes sense when you think about it. After all, lets say you have a world where the most powerful magic item in existence is a +3 sword. You introduce a new expansion or some brand new content to make your long-standing players happy (the newer players are still fresh enough to enjoy the old stuff). A lot of these players already have +3 swords. Are they going to be happy with the introduction of a new +2.5 sword? Of course not. They are going to want some new, cooler, more awesome items and powers for their characters!
So the new content includes an uber +4 sword! Very awesome! The players cheer! They upgrade! But now there's a whole bunch of now-useless +3 swords that USED to be the awesome sword of the game. But now they are junk. So the high-level players sell / give these swords to all the lower-level players who are struggling to kill vorpal bunnies with rusty knives. So now the vorpal bunnies are no longer a challenge, which means players are able to level up through all these challenges that were never designed for "twinked" characters very quickly.
Repeat this a few times for a mature world, and you get an economy and a power-level that speed of progress that was unimaginable to the original players.
A few months ago, Mike Hensley discovered that this was not limited to online computer games. Performing a simulation across multiple editions of Dungeons & Dragons, he compared the performance of a first level fighter against a series of goblins in one-on-one fights to see how they'd fare.