Houses in Harry Potter are more than just about family, so it would be obvious that, depending on the personality, the houses can or will be different. Maybe because a Hufflepuff and a Hufflepuff raise a child, the child could act pretty similar by the parents teaching needed traits to the kid. So it would be a higher chance for the kid to be in the parents' house. Raven + Griffin = more likely to be Ravenclaw and Gryffindor. That's what I think, at least.