The article only compares Windows hosting numbers. It does not include Linux. I <i>think</i> for Amazon the number of Windows hosts is relatively small compared to the total hostings...<p>A typical vendor-sponsered research result :-)
It appears that this is mostly due to the growth in Azure, and that <i>that</i> growth is primarily due to Microsoft cloud-hosted services using Azure. (The article does state that part of the growth is due to internal MS stuff, but does not give any indication about the relative amount of MS machines vs third party users' machines.)