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Various parts of our algorithms can apply to sites at different times, depending on what our algorithms find. [google.com...]
It's useful for me to combine that tidbit with another big Google topic we're also discussing Traffic Shaping [webmasterworld.com]. If Google is not yet sure where a site should be "profiled" it may be tested and ranked according to a different algorithm from time to time. In that way, rankings could drop significantly, even though there is no penalty.
A shift in the opposite direction is also a possibility. Google changes a site's profile and all of a sudden it "comes from nowhere" to rank on the first page.
(Thanks to SERoundtable [seroundtable.com] for spotting JohnMu's comment)
[edited by: tedster at 2:25 pm (utc) on Oct 30, 2010]
[edited by: Kristos at 4:29 am (utc) on Oct 30, 2010]