Their disclaimer explains it:
"W3Schools is a website for people with an interest for web technologies. These people are more interested in using alternative browsers than the average user. The average user tends to the browser that comes preinstalled with their computer, and do not seek out other browser aternatives.
These facts indicate that the browser figures above are not 100% realistic. Other web sites have statistics showing that Internet Explorer is a more popular browser.
Anyway, our data, collected from W3Schools' log-files, over a five year period, clearly shows the long and medium-term trends."