It's a simple matter of mind games, as previously stated. If Player A uses Olimar and Player B uses MK, the character match up is assumed to be 50-50 (or it should be, at least). Now, if Player A goes into a tournament, and faces Player B, the match up isn't 50-50, because the characters are not mindlessly fighting each other, the players use mind games. Each and every game that Player A faces an MK he will have quite a strong advantage over the MK, because his mind game ability will almost always surpass that of an opposing MK. Olimars are always working on their mind games, for this particular match up. Always testing it and perfecting it. An Olimar with such training can easily have a 60-40 vs. your average tournament placing MK. Why? Because MK isn't training to out mind game Olimars, they are too worried about Snake, DDD, Falco, etc. Now it is true that some MKs do in fact practice mind games vs. Olimar, but that has its downfalls. First, they have to gamble, they have to decide whether Player A is an Olimar that mind games well, or just your average Olimar. If they decide that Player A is just a casual/competitive player, or get cocky he will take an easy 40% early on. If they decide correctly, then oh well? You lose your advantage and it becomes the 50-50% match up that would have been expected anyway, no big deal.
Just my opinion, anyway.