Well imagine a melee list where a hypothetical character had 100:0 matchup against all characters below ice climbers and a 0:100 matchup against all characters ice climbers and above. When the match starts against any of the top tiers he just automatically SDs 4 times without any control and when he is against mid-bottom tiers the other character automatically SDs 4 times without any control.
Under this system, that character's matchup rating would be about 69%. Fox's matchup rating (at least according to SSBwiki) is about 68%. That means that even though said hypothetical character would be physically incapable of winning against any of the characters that the vast majority of people play in tournaments (and the others could just switch), he would be number 1 on the tier list, when in reality, such a character would be the worst character in the game.
Now obviously this is a hypothetical taking things to extremes but in a realistic setting, a character could move up or down several spots based off of polarizing matchups.
To further illustrate this point, look at the matchup averages on the matchup chart on SSBwiki:
http://www.ssbwiki.com/Character_matchups_(SSBM)
Under this, Ganon and Samus should both be higher and Yoshi should be grouped with bowser and kirby, and those matchup averages are rounded. If you don't round them, then a lot of characters move around one or two places.
I know this chart is outdated, but it is simply being used to illustrate a point.
EDIT: Also that chart was started yesterday so it isn't really stable at the moment. For example, since the first time I looked at it around 3 hours ago, jigglypuff has moved up one, Link has moved from 6th to 4th, Sonic has moved to 10th from 11th, (and the inverse to marth), Falco dropped from 4th to 5th, and Donkey Kong rose a spot or two (not entirely sure).
