anteck - 6:13 am on Feb 6, 2011 (gmt 0)
However, that theory doesn't fully explain why the traffic would differ in quality when rankings appear to be the same. My best guess is that it's the "user type" taxonomy that is changed - and that axonomy might be geographic, or degree of technical savvy, or native language - or lots of other possibilities.
My thoughts exactly Tedster. I had this random insight last night in the midst of falling asleep.
If google are able to profile users, then this would explain a lot.
Zombie traffic would be google expanding the subset of searches out to what it thinks 'might' be a good profile match. With a Algo change, this matching technology could be reset somewhat, so G tries sending different results out to different profiles, eventually matching up the most successful searches with the most responsive profiles.
Zombie traffic is G in the 'search & match' early stages of finding the best profile match.
We would never see this in action, however, with rank checking, etc, or being logged out, as G has no reference to any profile or search history. When we are logged in, out profile would match up well as we usually visit the sites we work on= and give good signals that we like that particular 'site'
It's a very fitting theory...