I think that part of the difference you get from different samples is at least partially based upon how much you weight Europe.Those W3 numbers are really low and don't jibe with Statcounter and NetMarketshare.
and they sift through all www browser strings to identify the platform:BROWSERS: What browsers do you track?
To see the list of browsers currently tracked, please select the "Browser" statistic and choose the "Bar" graph option. Next click "Download Data" - the downloaded file will list all browsers currently tracked.
http://gs.statcounter.com/faqMOBILE: How do you track mobile devices?
We track over 15 billion pageloads per month across the StatCounter network of over 3 million websites. We analyze every hit in order to gather as much information as possible. With each hit a useragent string is sent which allows us to determine the the browser and operating system used and also to establish if the hit came from a mobile device. In this way we determine the usage of mobile devices, browsers and operating systems.