Apple's developer support program tracks the iOS version in use on iOS capable devices. That data is posted for developers to look at. Currently, 90 percent of iOS capable devices are running iOS 7.
Here's the diagram posted at Apple's developer website.
Information like this serves several purposes. First, it helps Apple iOS developers size up the stats on the general audience so that when they build their apps, they know what the expected OS is on their customer's devices. That helps with decisions about what APIs and features to invoke.
This kind of information also serves a secondary purpose. It demonstrates the customer enthusiasm for Apple products. Apple is never shy about showing developers how much their products are embraced, the enthusiasm for the latest iOS release and the business prospects associated with developing for iOS.
Finally, this data reinforces the notion that Apple has been wise in its roadmap and hardware design so that fragmentation is minimized. That also helps new developers see a case for iOS development without concerns over fragmentation. OS fragmentation has been a problem with the Android OS and often forces developers to develop many different versions of their apps for each OS release.