Why we can use type definitions in the private part in class declaration

Question

It is considered that those definitions (members of classes, types etc) that are in the private part are data and types of the class itself. But there are exceptions that I don't understand. For example, why in the example below, do the C++ language rules fail to describe the type with the keyword auto?

class MyClassA 
{

    class MyClassB
    {
    public:
        MyClassB::MyClassB() {};

        MyClassB::~MyClassB() {};

        void get() const {};
    };

    MyClassB m_B;

    public:

        MyClassA() {};
    ~MyClassA() {};

    const MyClassB& get()
    {
        return m_B;
    }
};

 int _tmain(int argc, _TCHAR* argv[]) 
 {
    MyClassA classA;

    const MyClassA::MyClassB& ref1 = classA.get(); //! error C2248: 'MyClassA::MyClassB': cannot access private class declared in class 'MyClassA'

    const auto ref2 = classA.get(); //! OK, but why is it good here?

    ref1.get(); 

    return 1;
}

What does the С++ standard say about this?

I am developing code using MSVS2015 (toolset v140)