带指针容器的自动类型推断

我不明白为什么编译器决定在第三个for循环中为迭代器itr使用指向in的指针,而不是像前两个循环那样使用指向指针的指针。我试图分析 the deduction rules 但找不到相关部分。

void foo()
{
    vector<int*> a;
    int b[] =   { 1, 2, 3 };

    for (int i = 0; i < 3; i++)
    {
        a.push_back(&b[i]);
    }
    for (vector<int*>::iterator itr = a.begin(); itr != a.end(); ++itr)
    {
        //itr is a pointer to a pointer to an int
        cout << **itr << endl;
    }
    for (auto itr = a.begin(); itr != a.end(); ++itr)
    {
        //itr is a pointer to a pointer to an int
        cout << **itr << endl;
    }
    for (auto itr : a)
    {
        //itr is a pointer to an int.  Why?
        cout << *itr << endl;
    }
}