IUnityContainer container = new UnityContainer();
I read this code as: create a new instance of the UnityContainer object: that object implements IUnityContainer. By definition, you cannot create an instance of an interface. So, in this case, you are not "initializing an object from an interface."
UnitContainer container = new UnitContainer();
To understand how this invocation differs from the first, I'd need to know: what are the differences between UnityContainer and UnitContainer ? I'd speculate that UnitContainer does not implement IUnityContainer.