Windows doesn't matter to Microsoft now. The big sellers are Azure and Office 365 and they work on Macs.
As for market share in the US, I very much doubt it's that high really. There are huge unimaginable swathes of windows boxes which won't show up in GlobalStats at all. I know a company with 50,000 desktops that can only visit a web site on an internal network for example. Those sorts of installations are everywhere.