[epi0Bauqu]

Yes, what you said sounds plausible and is indeed intuitive, but we've found it empirically to be untrue.

In fact, on fresh private mode browsing (cookies cleared) when people searched for the same result at the same time, we still saw significant variation in results, even in the same country and on non-location results.

[moultano]

This may have nothing to do with personalization. http://googleblog.blogspot.com/2008/09/search-evaluation-at-... "We also make use of experiments, in which small fractions of queries are shown results from alternative search approaches."

Many small experiments can lead to differences for a reasonably large fraction of queries.