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If you just search for hardware store, in a search engine world tailored to the 'most `relevant`, most linked-to, etc' you probably could expect to always see the larger ones, the type with the big blue sign, or the big orange sign? The rotation might allow those searching for the lesser-known to have it come up first.
Especially in highly competitive areas, where there are more than 10 or 20 with the same PR, those would be easy to 'randomize' a bit.
Occasionally even us commerce sites try to buy something.
When we tried to buy a dedicated server, one of the top results was a directory that returned a page about servers - with no links to servers on it.
Oh well! That must be progress.
(Does anyone remember the days when if you needed more RAM for a computer you simply typed it in - and Google came up with an expert site? I must be getting old - but complete crap is now served up by the self-proclaimed masters of the Internet because they want to line their pockets with gold.)
Then when I tried something like "pc100 ram" it all seemed to come up good. Good SERPs, good adwords, good ads at the top of the page, and good froogle entries.