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As far as I know, they simply randomize them...which, imho, makes for a terrible surfing experience.
Though perhaps if you pay $$$, you get stable spots in the SERP's?
Inktomi - now owned by Yahoo, utilizes link popularity, & on page factors to sort web pages - and naturally, uses the link map of the web as well.
Unless you meant the 'rank for your pages' fluctuates? Yes, that is correct - I have no idea why, other than to confuse people who are not paying per click, perhaps, as that is the largest potential revenue stream for their index.