As text files, cookies may be deleted from time to time. If the cookie for a particular website is deleted between visits, a browser is issued a new cookie upon its return. In such cases, the website will have recorded two cookies for the same individual browser and the resulting cookie count is actually larger than the number of browsers that have visited the site.
Adding complexity to this problem, while many people may use the same browser to access a website, others may use multiple browsers or machines (at home and work, for example) to access the same website. This creates a more complicated environment and is why Quantcast has developed a sophisticated model to deal with cookie deletion and multiple machine usage to produce accurate estimates of the number of people that visit web destinations.