Without even clicking the link, my guess is that it is because of creative ways of saying what it means to have "sold" a license. Clearly, Windows 8 has fewer than 60 million users.
Demand a more concrete number from Microsoft. How many people have a valid credit card number on file on the Windows 8 App Store? How many apps does an average user buy? What is the average price of an app on the Windows app store?
1700 devices certified means nothing to me. Show me the money.