Str82u - 10:36 am on Dec 9, 2012 (gmt 0)
The big problem I have with this theory is that surely it's been going on too long for that. It seems like as long as some people have been seeing this on their sites, Google would have learned everything they needed to know.
I've been suspecting this for all results but in my mind it was more like Google temporarily rearranges the SERP to test if the lower ranked sites perform better (to them). With the way changes have been in the last 2 to 2.5 years Google still has a lot to learn. SERPs fluctuate for us between one set of long-tailed phrases and another that is effected mostly by locations; this is weekly. There are other times that it happens with more stable, higher traffic phrases but only for a few days then everything settles back down.