There has been discussion at SEO Chat forums in a thread named Algo Merry Go Round??, where members discuss the large deviations between rankings for the same keywords among different keyword sets. For example, a standard keyword checked at Google's various data centers (using Datacenter Quick Check tool) can rank between position 57 and 132. In the past, this was not really normal. There was flux between the datacenters, but there seems to be a much greater flux these days, on a more frequent basis then before.
So the theories are: (A) AdWare is causing the problems (B) Google is testing different algorithms at different datacenters (C) There is nothing wrong at all.
My question to you, which one you think is right? A, B, C or None of the Above?