Performance of "direct" virtual call vs. interface call in C#

This benchmark appears to show that calling a virtual method directly on object reference is faster than calling it on the reference to the interface this object implements.

In other words:

interface IFoo {
    void Bar();
}

class Foo : IFoo {
    public virtual void Bar() {}
}

void Benchmark() {
    Foo f = new Foo();
    IFoo f2 = f;
    f.Bar(); // This is faster.
    f2.Bar();    
}

Coming from the C++ world, I would have expected that both of these calls would be implemented identically (as a simple virtual table lookup) and have the same performance. How does C# implement virtual calls and what is this "extra" work that apparently gets done when calling through an interface?

--- EDIT ---

OK, answers/comments I got so far imply that there is a double-pointer-dereference for virtual call through interface versus just one dereference for virtual call through object.

So could please somebody explain why is that necessary? What is the structure of the virtual table in C#? Is it "flat" (as is typical for C++) or not? What were the design tradeoffs that were made in C# language design that lead to this? I'm not saying this is a "bad" design, I'm simply curious as to why it was necessary.

In a nutshell, I'd like to understand what my tool does under the hood so I can use it more effectively. And I would appreciate if I didn't get any more "you shouldn't know that" or "use another language" types of answers.

--- EDIT 2 ---

Just to make it clear we are not dealing with some compiler of JIT optimization here that removes the dynamic dispatch: I modified the benchmark mentioned in the original question to instantiate one class or the other randomly at run-time. Since the instantiation happens after compilation and after assembly loading/JITing, there is no way to avoid dynamic dispa