I'm guessing that similar to the "top news" newsfeed (which is even more of a default now) there's some "magical" algorithm in play there as well that FB thinks is useful somehow when ordering friends for is (and not even providing an option to sort them differently of we wanted to).
Probably just like with top news there are various factors in play and no one really knows for sure (except those who work on it and likely can't talk about it anyway).