IanTurner - 1:12 pm on Mar 10, 2013 (gmt 0)
Have to stress this is just my theory. Have been scouring the web for info on whether they ever reset personalisation and how and why, but came up with nothing. Seems like the SEO community isn't interested in it even though it plays a part in sending traffic. Anyone who has info to chip in, please do!
There is definitely something in what you say - I'd guess a lot of personalisation comes from browsing history and you can only store so much against a Google account - before it needs wiping to make more room. Local history will be wiped when someone clears cache, so if they are not logged into a Google account cache clearing will change personalisation.
@tedster - I think that this deserves its own thread.