leadegroot - 10:35 am on May 31, 2012 (gmt 0)
Could it simply be actual traffic throttling?
I mean to share the results, rather than attempt to push people to adwords
eg There are 20 decent sites that satisfy query A, so they get rotated around and each get a turn at the 1 - 5 slots.
Its a shame WMT isn't more specific in the data it returns. :(