A few times a month, we're asked "why are my stats different between Urchin and Google Analytics?" If Urchin and GA are looking at the exact same data and there's a numbers discrepancy, 95% of the time it's caused by the visitor tracking method in use.
In order to understand IP+UA, have a look at a single hit in a web server log file (below):
220.127.116.11 [01/Jan/2005:12:43:20 -0800] "GET /milw.html HTTP/1.1" 200 678 "http://home.earthlink.net/~milwaukeeroadcoastdivisiondvd/id7.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0)"
For the above hit, the fields (L to R) are:
client-ip, datetime, cs-request, sc-status, sc-bytes, cs-referral, cs-useragent
With the IP+UA tracking method, Urchin picks out the client IP address and user agent (client-ip and cs-useragent) from each hit in the log file and uses the info to calculate a visitor session. IP+UA is easily foiled by proxy servers, firewalls, NAT devices, and the like -- if you have 300 visitors from behind a corporate firewall who appear to use the same IP address and user agent, your numbers usually won't match up with the true stats. This means the IP+UA tracking method tends to overstate traffic by a wide margin. A ~30% increase (compared to UTM) is average, although we've seen it as high as 400%! That said, you can use an exclude filter to "trim the fat" and get rid traffic from robots and spiders -- doing this usually helps make the numbers more in-line with UTM metrics.
The UTM tracking method is used by both Urchin and Google Analytics, but only Urchin can use the IP+UA method.