The web page actually says "Android devices that have accessed Android Market within a 14-day period ending on the data collection date". Most people load apps from the Android Market when they first purchase a device. After that, they only occasionally dip in to see if there are new apps. I know some (typically busy) people who never visit the Market again unless they are looking for specific app. Hence, just showing a "14-day period" probably causes the actual number of older devices to be under-represented.
Does this matter? If you are a typical developer creating an app, visitors to the Android Market are the way your app will probably get discovered. Hence, your target market is the same as the chart. However, if you are a large brand or if you have some other way of marketing your app, your target market is probably includes those people who don’t visit the Market any more and you might have to make an allowance for a larger proportion of people with older phones.