Actually, they're not. There's a problem with this chart, and it's one that has escaped several developers I've talked to:
It's based on tracking Android devices that visit the Android Market.
Not all Android gadgets can access the Marketplace, so this chart doesn't actually reflect the real world market shares of the various versions of Android. I can't tell you how many Android devices aren't being counted, but given the growth of tablets I'd say it's at least a third and possibly as much as half.
But I don't really know because answering this question is like asking how many birds did I not see today. That which isn't measured cannot be counted.