Low coupling theory vs reality [duplicate]

Coupling is defined as the knowledge one object has about another one, which describes how dependent they are. The more dependent, the worse, since changes in one would impact in the second. There are different coupling types:

http://www.principles-wiki.net/principles:low_coupling

Let's assume we talk about call coupling.

In Java, when A creates an object B and calls one of its methods it's said to be tightly coupled. But if we create an interface IB, used from A where B implements IB it's said to be loosely coupled. I don't see why since one change in the interface would have an impact in A and B. And one change in B would have an impact in IB and A. They still seem to be call dependent.

The same rationale applies to the Facade GoF design pattern. It's said it promotes low coupling, since we put an intermediary between a subsystem and the client code. In this case, it looks like we transfered the problem from the client code to the Facade. Since a change in the subsystem would have an impact on the Facade instead of the client code. The client code is no longer coupled to the subsystem but the Facade is.

I don't see how coupling is reduced.