Panthro - 2:31 am on Apr 11, 2013 (gmt 0)
My guess is that Google is actively comparing the user metrics between the "results" and put all sites into a battle arena. Sites that deliver "comparative" worse user metrics will lose traffic gradually. While the winner grows and eventually dominate the SERP against the loser sites.
This makes a lot of sense to me given the trends and the information we have about where Google is headed into the future. They want to give answers not options.