Log File Analysis
If a returning visitor has file caching enabled on their web browser and usually that is the case, the request for the file they have already viewed may not be sent by the browser to the web server, instead they will view a local copy, therefor AwStats or Webalizer will not detect multiple visits by some users. Also lets say there are four computers all on an internal network if they all visit your site they will all be seen as visits from the same ip address and therefore the same user by AwStats or Webalizer.
Both will also vary because the point of data collection is different for Google Analytics the data is collected on the visitor’s browser. We know how many of those there are out there with their countless configuration, version, platform and addon possibilities. For the others the data is collected in one single place, your web server. In fact, all three may even collect daily data in different time zones so ‘days’ begin and end at different times. If you host your site in a different time zone to the one you live in. Your Google Account will most likely be in your local time zone, and your server based reports will be based on the server’s time zone. These differences are partly responsible for the variations in the Analytics produced by the technologies.