A lot of the net is behind proxy servers.
When someone hits a page from MSNTV, for instance, they typically get the HTML with one server and the graphic at the top of page with another server.
Of course, when someone else from WebTV hits your site an hour later, your site never finds out about it - they serve up a copy that MSNTV cached, unless you prevent caching by attaching a cookie or using some sort of cache control header or meta tag.
AOL is behind caching proxy servers, too, but unlike MSNTV, they consider cookies to be part of the file, and they will give the cookies to parties they weren’t intended for. Never overestimate the intelligence of AOL or Microsoft.
And then there are corporate firewalls. A good deal of my traffic appears to be bored workers on 3-hour coffeebreaks. Corporate IT generally has trouble staying out of their own way when it comes to the net; they’re more concerned with keeping the sound card in the CEO’s PC functioning correctly.
Since raw log files are readily available, it’s no big deal to write a Perl script to parse them and generate whatever statistic you want. Within limits, those statistics may even be useful to you and to others.
Just remember, the map is not the territory.