To quote from there "...This page provides data about the relative number of active devices running a given version of the Android platform. This can help you understand the landscape of device distribution and decide how to prioritize the development of your application features for the devices currently in the hands of users...".
Software Architect, Developer, Book Author
While that page is undoubtedly useful, it needs to be said that it represents devices that access the Android Market - in other words, those from manufacturers that have chosen to license the Google Apps (like Maps, Navigation, Android Market, Google Talk, etc.) from Google. There are now more and more devices coming to the market that do *not* have those apps, and which consequently are not tracked by this page. I think this is especially true for devices sold in some Asian countries, and for some of the Android tablets. Just something to keep in mind when looking at those charts.