I think they just tend to pick familiar faces off the crowds first then look around and see who else is left.
At the end we still made the finals but we might as well get dropped in the first round.
It was that close !
I've seen competitions where they have a 'last chance' round. I think its a great idea. They pick most of the quarterfinalists then the couples that did NOT make it get another round and they pick the rest. That way if someone was missed they can get pulled out of the crowd.