I'm pretty sure you haven't read the statements right before the table:

W3Schools.com is for people with an interest for web technologies. This fact indicates that the figures below might not be 100% true for the average internet users. (...) Anyway, our data, collected from W3Schools' log-files over ten years, clearly shows the long term trends.

But if you're not convinced, you can read in their forums:

    - The w3schools Browser and OS statistics are based on the data
      collected over the w3schools site or do have more sources? 
    - Only the W3Schools site.

http://w3schools.invisionzone.com/index.php?showtopic=35566

By the way: http://w3fools.com/