Compete.com only tracks the United States traffic to websites. We have worked for many companies assisting them with determining the discrepancies between multiple tracking services. Most everyone uses the free services such as Alexa, Compete, Quantcast, and of course paid services such as Comscore. Typically these four services are compared to a websites statistics provided by a service such as Google Analytics.
When comparing actual site analytics to these free services you are bound to see discrepancies. By actual we mean tracking of every page as opposed to statistical sampling which then infers the amount of web traffic or pages viewed to a site.
I wanted to point out that Compete only provides web traffic data from users in the United States. Just knowing that will answer most of the initial questions regarding discrepancy in compete data with your own.
You should also know Compete’s methodology for determining web traffic to a particular website. While Google analytics provides code to put on every page of your website in an attempt to obtain actual data, Compete uses a panel of 2 million individuals. The panel is a cross section of Internet users in the United States and they obtain click stream data from ASP’s and ISPs. They also obtain data from widgets, applications, and their toolbar.