In the iOS 7 TechTalk, session Architecting Modern Apps, Part 2, they explain this clearly
Good read Hi! I’m #available!
So, a modern App might use iOS 9 as the Target SDK, and iOS 7 as the deployment target. This means that you can run on iOS 7, iOS 8 and iOS 9, and that you have available to you any iOS 9 calls when actually running on iOS 9.
You can read more in my post SDK and Deployment Target