makes the odd good defense play, does that mean it's wrong to say he's generally a bad defender? If Dwayne Wade scores 15 points in a game, does that mean it's wrong to say that he generally doesn't score close to 30 a game?
But this is sports, generalizations apply just as well to scientific fields like genetics, psychology, chemistry and Quantum Mechanics, science does not rest on absolutes, it rests on statistical probabilities, it makes statements about what generally occurs, which can be demonstrated repeatedly in the real world.
The whole world functions by using generalizations, this because things generally happen a certain way, always with rare exceptions.
So when I make the generalization that custody of the kids goes to the cheating or non cheating wife who gets child support, it's because that's what generally happens, despite relatively rare exceptions.