I found this explanation,
* Google Analytics and other programs like Awstats will not track our visitors and should not be used to track 'paid traffic'. For example, Google and Awstats have no way to detect whether traffic is coming from a redirect or not. The main reason they dont pick it up is due to cookies. As a redirect, cookies are considered 3rd party. They can not read 3rd party cookies due to default browser settings. This is common with most paid traffic. We recommend you use your client panel that is associated with your account. If you want to use a 3rd party we recommend you only use www.Statcounter.com or Raw Server Logs which are the most accurate method for traffic comparison.