Originally Posted by instaunt
So how is Google Analytics tracking almost everything?
It isn't. There are lots of groups of visitors that it doesn't track at all.
For example Analytics doesn't track any of my visits to any pages anywhere on the web that I go to because I blocked their site so as to speed up the download of the web pages - since there is no benefit to me to have my visits tracked by Google.
Also the referrer is as inaccessible to Analytics as it is to you in those situations where Analytics even knows that the visitor exists.
Overall you can track more about your visitors than Analytics can because of the information logged on your server that Analytics doesn't see.