We have Piwik running as well as Google Analytics on our sites. We’ve noticed that the Piwik figures are massively higher than the GA numbers.
If we take Googles “users” number for today it reports 516. If we look at Piwiks “unique visitors” for today it says 1538. That’s an additional 1000 visits.
Now, it might be that “users” and “unique visitors” are different metrics, but from what I can see they are broadly similar. Certainly there are no Piwik metrics being shown in the WP portal that match up with the GA numbers remotely.
You should compare unique visitors to unique visitors for a comparison analysis of teh 2 programs. (also try pageviews)
Also I find when people comare GA to piwik often times they miss pages with code one way or the other…
I recommend you take a 24 hr period for your site.
Then do a pages report for it one in GA and one in piwik, then compare results…(is there a big discepancy in actual listed pages measured between the two?)
Then pick actual pages one to one from GA and Piwik and see what are comparison results from there…