A ditto is just another matchup. At more intermediate levels, players will have more varying results in different matchups (lack of knowledge of the matchup and stuff like that come into play). If I main fox but never play against other fox players, how am I supposed to know the matchup? Knowing a character's moves helps (you will know your own character's moves), but it still won't make you good at the matchup.
At higher levels, when players are more well-rounded, matchups matter less, but it's still the same thing there. It's just more obvious at lower levels because the difference can be quite obvious.
There's also just players that beat other players. Sometimes you play someone so much that they go even with you, but then you both play against random other people and one of you performs way better. Then there are cases where your just not used to how your opponent plays (super campy, for example), so you lose because you are unfamiliar with what you are playing against.
The way it is usually measured is placement across many recent tournaments. This way, it measures their overall skill against many opponents. It could still easily be inaccurate, but it is one of the best measures there is.
