Like perhaps one set of parents consist of an orphan and someone who grew up in a household with mentally-ill parents, while the other set come from comfortable intact middle-class nuclear families? And the first set just don't know how to parent because they had shitty or no role-models growing up?

I would still respect the first set of parents. They did the best they could with what they had to work with. It's a sad situation though...in a perfect world effort would equal results, but the world ain't perfect.