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When the light is RED, do you notice any significant position changes in your B & C sites? I don't. This is were the is a complex paradox. SERP's "normal", traffic junk.
However, that theory doesn't fully explain why the traffic would differ in quality when rankings appear to be the same. My best guess is that it's the "user type" taxonomy that is changed - and that axonomy might be geographic, or degree of technical savvy, or native language - or lots of other possibilities.
[edited by: tedster at 5:42 pm (utc) on Feb 17, 2011]
[edit reason] changed the link [/edit]