Read up what these are:
http://bulbapedia.bulbagarden.net/wiki/Effort_valueYou might have been under the impression that a Pokémon you caught was stronger than one you traded from someone else, after raising them both to level 50 or so.
In fact, the Pokémon from the other trainer just got less of these EVs because it needed to defeat less enemies to reach that level.
Used to be a common misconception because EVs had never been referred to in any official guides at that point.
