The figures are based on Google's own data; the company routinely tells developers the percentage of devices hitting the Google Play Store running on each Android version. The latest figures were taken over a 7-day period ending on September 7.
The adoption rate for the latest Android software sorely lacks behind that of iOS, although with iOS 8 last year, Apple's own adoption rates were lower than in years past.
The main reason for the sluggish upgrade numbers was a lack of free space available on some iPhone models. Apple says it won't have that problem this time around with iOS 9.
While Google Play Services have helped mitigate Android's version fragmentation, it doesn't completely resolve the issue.
Numerous developers have reached out to me citing that they're still constrained by the new APIs Google introduces with each version of Android.
Google Play Services can only help with that challenge so much, leaving developers to target more recent handsets with their apps. Perhaps that's another reason developer interest in iOS 9 is greatly on the rise in certain circles.
Google expects to launch Android 6.0 later this year -- possibly later this month with a pair of Nexus handsets -- and the upgrade cycle will begin again. With handset makers and carriers slowing down the process, I suspect the adoption figures next year will follow suit.