The subject asked a very good question! There is indeed a problem that the corresponding resolution function is different. But this difference is often allowed to be ignored in this kind of problems, because the positions of X and Y in f(x, Y) are equivalent to each other and the relationship is independent. Taking the original title as an example, if U corresponds to Y and V corresponds to X, the dz obtained only switches the positions of X and Y in the result, which has no influence on the investigation ability.