different results from AWStats and Google Analytics

I found different results from Google Analytics and AWStats web analytics, and just did not notice that before today. I was happy to see the results from AWStats and at the same time worried by seeing the results from Google Analytics. AWStats provides over counted statistical data whereas it seemed that Google Analytics was not providing all the data.

I am thinking that following might be reasons for difference in statistical results from Google Analytics and AWStats:-

1. AWStats calculate the visitor for less time span where as Google Analytics calculate visitors for long time period thus reducing visitor count.
2. Automated spams, robots have affects in executing the AWStats whereas Google Analytics statistics are not affected.
3. Other java scripts interfering Google tracking code and turned off cookies would trouble Google Analytic for tracking results.
4. Google Analytics counts the results only if the pages are loaded fully. Navigating away without pages being fully loaded means data is not collected for that action as tracking code would not be able load to operate.

Also, I got something from Google Analytics Help Center,
here's the link: https://www.google.com/support/googleanalytics/bin/answer.py?answer=55614&hl=en_US