Unique visitor overstatement is a well known phenomenon in web analytics based upon site-centric methods.
Unique visitors overstatement is due to cookie deletion and cross-device browsing.
If a users visits three times a web site within a month and deletes his cookies, he is commonly counted as three unique visitors. The bias is usually the same if a user visits the site from three different devices.
A 2009 comScore U.K. study has estimated that a server-centric measurement system using first party cookies will typically overstate the true number of unique visitors by a factor of up to 2.4X.
It may be thought that with an increase of cookie deletion and the rise of cross-device navigation, the overstatement is much worse.
Unique visitor overstatement can be reduced with cross-device identification.