Google sees how people followup queries over time. So they know the relative frequencies of the four distinct progressions: [Obama], [Iran], [Obama Iran] [

Google sees how people followup queries over time. So they know the relative frequencies of the four distinct progressions: [Obama], [Iran], [Obama Iran] [Obama Iran] [Romney], [Iran], [Romney Iran] [Romney Iran] The triggering could be as simple as the fact that {[Obama], [Iran], [Obama Iran]} happens more often than {[Romney], [Iran], [Romney Iran]}. And that difference could be because people searching one path are slightly more likely to keep refining simple queries ("try, try again"), versus other people more likely to combine-up-front ("measure twice, cut once").