Like you said, it has something to do with EVs, it doubles them to be exact. See, everytime you faint a Pokemon, you get EVs. Each Pokemon can get a max of 510 EVs. It's kind of complicated, so I would read the EV guide on the Diamond Board but the gist of EVs is that it makes your Pokemon stronger through a specialized training than basic training.
---[insert signature here]--- ...Official Nuzleaf of the B/W boards
EVs are a point system that helps increase the stats of you Pokemon at level 100. Each time you defeat a Pokemon, you get an EV in one or more stat. For every 4 EVs in a particular stat, you will raise that stat by 1 at level 100.
Say you have two identical Charizards, and you raise one to level 100, and it has a speed stat of 250. Then you take the other Charizard, and EV train it against Magikarps (yes, there are in fact useful here), and get it the maximum 252 in Speed EVs. Now when you raise it to level 100, it will have a speed stat of 313.
This can be a bit difficult to understand in the beginning, but after you get the basics down, it just takes time to EV train your pokemon.
GLAD GANG l SSS l No.XII Oh! The shame, the humility, what a wicked gang are we! Not changing this sig until Si comes back B/W forums 9/3/10
It's easier experienced than explained. Most of Pokemon's intricate mechanics are hidden, as they're not about making the game directly complicated to understand for their younger demographics, so they make a large influential bulk of the enhancement system hidden.