I'll just add one more observation. I add new pages to my site on a regular basis. It usually takes ten days for Google to index a new page, sometimes as long as three weeks, sometimes just a few days. It usually takes 2-3 days from the time a page first appears in the index until it reaches its eventual traffic peak though this has at times taken weeks.

During those transitions the new pages exist on some servers in some data centers and searches on those keywords are often different with each query.

Multiply that by the number of people adding new pages to the internet. What is amazing is not that the results are sometimes different but that they have as much consistency as they do.

I expect more stability as the big daddy infrastructure settles in but the index will never be "finished."

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...the Rockies may tumble, Gibralter may crumble... G & I Gershwin, 1937