Macs and PCs might have different lifecycles that might explain some of the difference; if Macs are used on average twice as long as PCs then 10% of shipments might turn into 20% of running computers. Furthermore I suspect far more PCs end up in places that never browse web and become counted by statcounter but are counted in shipments.

The 10% is its share among the computers shipped to customers whereas the 22% is its share among requests (term?) made by desktop OSes to web servers.

These figures are consistent if we assume that the average Mac makes 2.2 as many requests to web servers as the average desktop computer does--which I am ready to believe: some people use the web much more intensively than others do, and I can easily believe that such people are more inclined (specifically, more than twice as inclined) to chose a Mac than the average computer buyer is.

If a person does not care about computers or the internet and uses them only occasionally, then simply based on the differences in purchase price, they are probably much more differentially likely to buy a Windows machine than a Mac.