When visiting a webpage, the referrer (or referring page or referer) is the URL of the previous webpage from which a link was followed. Referrer tracking cannot be very accurate because of some technical reasons:
Due to privacy and security concerns, some of the modern web servers and browsers hides the referrer link (Referrer hiding). This is done by either making the referrer field in the HTTP request blank or adding false URL. So the destination browser takes the false referrer URL as the correct one without knowing it is fake.
Secure (Https, ssl) access
If a website is accessed from a HTTP Secure (HTTPS) connection, and a link points to anywhere except another secure location, then the referer field is not sent. This is done to add security and privacy for the end users who are browsing. Even using famous software such as Google Analytics, which is mostly based on source analysis, you can see that the referrer accuracy is (normally) less than 30%: most of the times, the referrer field is left blank.
So as a matter of fact, the reliability and accuracy of a referer tracking often varies. In some reports, you can see the referer field being empty and in other this filed will be full of interesting data.
ClickMeter do not rely a lot on referrers and allows you to use one tracking link (or pixel) for each of your main sources. In this way reports can be 100% accurate about sources, because you are deciding which is the source. You can be sure from where the visitors come.