John
My guess is its google testing different formats. Have you ever use google analytics they have a tool that shows the clicks on your pages. google must use this for their own purposes on search results.
My view is that the big change in the position of google maps is them trying to take more money on ppc, so they must be look at their own pages to see if this is working. by pushing the SEO listing below the fold and having not only the top three positions generating money but the top half of the page which is why i think maps will have a price soon attached to listings.
just think of all the money SEO experts take each year all on the back of google. if it was you i sure you would look at getting some back if times started to get hard.
Or, just like microsoft google have to give market share away. just think if google gets say 80% of all searches that's a very dominate position and governments don't like that. so they have to show that they are fair in some way.
All the above is just my own view based on stuff i read on the internet about this subject.
Respect
Ian Harper