Well, on second look Google actually doing very little in ad rotation, suppose if a certain ad is highly targeted to the content of your site, it might be shown several time. Secondly when you attract a large number of return visitors, or generating a high number of page views per visitor, you may experience decline in click through rates on the ads over time.
If you analyze it well it shows that Google targets the ads based on your site’s content, not your visitor’s desires.
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