That is only from those who visited the w3c schools website, so that skews heavily techy, so Linux will be disproportionately represented.
The one I gave, OTOH, is gathered from around 30 000 sites, IIRC.
Still, that 4.5% really looks nice
What I wanted to point out that statistics isn't really that reliable :P
The one you linked for example seems to be mostly US based so you'll get a view how the usage is there. And as it doesn't include all US sites you can't really be sure if you miss a group of people.