Something to bear in mind here...
Everyone is making sweeping generalizations. Now I usually like those - they're based on some kind of logic even if it is sometimes fuzzy. But you have to keep in mind that everyone is different, unique, won't necessarily fit into whatever bracket you intended to put them in.
You might decide to try dating "below" your punching weight in order to get your confidence up, but the girl that you pick out might have just had a string of abusive relationships and decide to stop taking any **** from the men in her life - so you move in with an air of confidence and pow, she shoots you down. You couldn't see it coming, but it's easy to see why assumptions don't always work out the way you planned... see?
I don't think there has ever been a hard and fast rule to dating. You can narrow it down some but it's not an exact science.