Letterboxing when tested in iOS Simulator using iOS 7.1 and iPhone 5s

I am having difficulty understanding why apps are processed by mailboxes when simulating iOS 7 on 5 and 5. When using simulated iOS 8, it is correct. What could be causing this? I don't have the 7.1 SDK on my computer, but I was told that I don't need it if you are using Swift (although I still face the problem on another computer when using the Xcode 7.1 SDK with Objective C and Xcode 6).

The figure below shows a default main part application with zero lines of written code. The only thing that has changed is the development target (for iOS 7.1). I checked constraints on views in storyboard and everything is checked.