That's an interesting statistic. While it's far away from being anything representative, the Windows Phone app "i'm a WP7" app unveils that only 57 % + 1 % of all the participating devices are upgraded to Windows Phone 7.5 aka Mango. As of today, "i'm a WP7" has 156,168 registered users so the results can't be anything representative but nevertheless, it clearly shows that there's a difference between the update availability and executed and installed apps. At least, the participating users are able to download and install apps from Microsoft's Windows Phone Marketplace and therefore they aren't total mobile noobs but were even able to create Windows Live IDs.
While it's not a problem of the Windows Phone 7.5/Mango update availability itself - according to Microsoft all carriers and manufacturers in the U.S. and rest of the world are delivering the Windows Phone 7.5 upgrade now - it could show a different but more fundamental problem. As a matter of fact, Windows Phones can only updated through a Windows PC or Mac. And while the Windows Phone itself notifies about available updates, the update has to be done through a PC/Mac which either needs Microsoft's Zune software for PC or Microsoft's Windows Phone 7Connector for Mac installed. However, not all mobile phone users are also using their PCs for this kind of tasks but I know many users which are using mobile- and smartphones but rarely use PCs other than their office PCs. This means that these customers are virtually unable to update the phone software and therefore they are stuck with the device's release firmware.
In my humble opinion this will become an even broader issue as more successful Windows Phone will become and as less tech-savvy people (think about Prada or Jil Sander customers) are buying a Windows Phone. Maybe it's time for Microsoft to redevelop the update mechanism and getting rid of updates through the PC/Mac but offering FOTA (Firmware Over The Air) updates instead, as also supported and used by Android and iOS.
Cheers ~ Arne