here there are methods with same parameter and return type.
and You are calling method of class not of interface.
"An interface contains only the signatures of methods, properties, events or indexers." - MSDN
So interface is just for signature.
In your case Compiler will only check that the class who implement the interface implements its all methods or not.there is no concern with calling a method of particular interface.
Generally we use interface in terms of polymorphism and to take Parent-Child reference related advantages.
Here i have answer one question which shows the real time example of use of Interface.
In polymorphism what is the practical use of using a parent class reference variable to point to child class object instance?
I hope this may clear your idea.
Hope This Helps.