Many ISP's these days are running Cache servers, to speed up the transfer of commonly accessed sites by their users. In addition to this, there are several national and international caching servers set up now that many ISP's are using in order to speed transfer across the world of common sites.

The advantage, obviously, is speed, the disadvantage, is that the tracking software has absolutely no way of seeing hits from a cache server.

For example, the big ones in North America, AOL, Prodigy, MSN, Earthlink, etc., all use caching servers. So, If I used AOL and visited www.xyz.com, then any of the other ten million AOL users who visited that site after me would load it from the AOL cache, and NOT from www.xzy.com.

Thus, even if www.xyz.com had ten million visitors, but they were all AOL users within whatever period of time their caching server uses to refresh, the administrator of www.xyz.com would see only a single hit...