What does it matter?
Anyone that has developed for Android in the last few years knows that from OS version to OS version there are not many major changes to the SDK. They do add new features, but for the most part, the majority of software out there doesn't leverage most of these new features. As such, they are written using older SDK versions. This means those application are compatible with 98% to 99% of the android ecosystem. Fragmentation only becomes an issue when you need to leverage something in the newer SDK's, and these days most of the time this is newer hardware features that the older devices don't even have. Each time a new version comes out, it supports running software built for older versions, its backward compatible.
Apple has its own fragmentation issues, the only difference is that Apple doesn't advertise it with nifty pie charts that everyone can see on the web. There are plenty of older Apple hardware that cannot support iOS 8, just go back a few iPhone (original, iphone 2, iphone 3), iPad (oringial) and iPod Touch anything before 5G) versions. The difference is Apple just states the hardware can't support it, says its old stuff and out of support, and urges you to upgrade to something better. So if you have an iOS 7 device and try to run an app built for iOS 8, its not going to work, aseptically if it needs some hardware feature that doesn't exist on the iOS 7 device. This is the same fragmentation as seen in Android. The scale is smaller because Apple is one company, selling a small inventory of devices, and it doesn't advertise a pie chart showing the breakdown of active device iOS statistics.