You may not be getting all the referer information you think you are getting. Some data may be completely lost depending on what browsers and search engines are used.
I got to this article because something called GLinkPing.aspx was doing something funky on our server.
Check this out:
http://www.webmasterworld.com/website_technology/3076218.htm
and this:
http://www.javascript-examples.com/track-outgoing/