When Apple announced iOS 9, they also revealed a new feature called app slicing, this feature was designed for developers and it would let developers create apps which used less storage space on specific devices.
Apple has now revealed to its developers that the new app slicing feature is not available in iOS 9 at the moment, this is apparently due to a bug in iCloud.
App slicing is currently unavailable for iOS 9 apps due to an issue affecting iCloud backups created from iOS 9 where some apps from the App Store would only restore to the same model of iOS device.
Apple has said that the developers do not have to do anything about this at the moment, when someone downloads one of their apps on a specific device they will automatically get the universal version of the app.
Apple also went on to say that the App slicing feature will be turned on in the future, although they did not give any specific details on when it will be available.
Source Apple