While testing Piwik, I found some differences between Google Analytics and Piwik stats. I’ve read some posts there and I saw that I’m not alone. More visits on Google Analytics then on Piwik.
I had the opportunity to compare on a very small trafic site, only 2 or 3 visits by hour. That makes the Apache log readable.
What I found is that Google Analytics considers some bots and search engines a legitimate visitors, when Piwik makes the difference.
I’m not sure that is the reason for every differences, but that explains all differences for this site.