Because they don't have the same criteria.
For a male to be ROM, he needs to have produced 4 Champions;
For a male to be POR, he needs to have produced 15 total wins.
A dog can produce 4 Champions, and yet not have 15 wins total;
A dog can produce 15 wins, and yet not have 4 Champions total;
Or a dog can have produced 4 Champions and 15+ wins total ... in which case, if a dog does this, then he is both a ROM and POR producer
Jack