I've never watched the show, but if it works anything like the mental model of it that I've built through cultural osmosis, I wouldn't be surprised if they were filtering for compatibility in some way before they finalize the selections. Closer competition makes for good TV; the network's essentially throwing away free money if a non-trivial number of the contestants give it up as a bad job before they can cause drama.
This is the earlier promised post about Dealing with a Major Personal Crisis. Please continue reading there but comment here.
The reasons for posting it this way are explained at the end of the link. I hope this approach does what I want it to.