Why exiting "using" invokes unexpected Dispose()?

Question

Here is my case:

class A : IDisposable
{
    public void Dispose() { Console.WriteLine("A Dispose"); }
}

class B : A
{
    public new void Dispose(){ Console.WriteLine("B Dispose"); }
}

class Program
{
    static void Main(string[] args)
    {
        using (B b = new B())
        {
        }
    }
}

Final output is "A Dispose". I've read some documents which introduce the difference of override and new, and something telling that "using" substantially equals to "try-finally". But I am still can't get answer to why it is not B::Dispose got automatically invoked. Comparingly following's output is "B Dispose"

B b = new B();
try
{
}
finally
{
    b.Dispose();
}

Please help, which did I miss.