Why IOCP doesn't work in BeginExecuteReader

See more:

C#

IOCP

asynchronous

I've read many articles say that IOCP is used in BeginXX/EndXX pair calls.
However, when i test them, my result shown that IOCP didn't work in BeginExecuteReader call, while it worked just fine in BeginGetResponse call.
I'm very confused with this outcome.
Can anyone tell me the reason?
Is there anything wrong with my test code?
Here's the test below:

1. test with BeginGetResponse
Code:

public static void IoThread2()
{
      ThreadPool.SetMinThreads(5, 3);
      ThreadPool.SetMaxThreads(5, 3);
      int w; int io;
      ThreadPool.GetAvailableThreads(out w, out io);
      int cid = Thread.CurrentThread.ManagedThreadId;
      Console.WriteLine("Begin:" + w.ToString() + ";" + io.ToString() + "; id = " + cid.ToString());
      ManualResetEvent waitHandle = new ManualResetEvent(false);
      WebRequest request = HttpWebRequest.Create("http://www.cnblogs.com/");
      AsyncCallback cb = new AsyncCallback(IOThread2CallBack);
      request.BeginGetResponse(cb, request);
      waitHandle.WaitOne();
}

public static void IOThread2CallBack(IAsyncResult ar)
{
     try
     {
         WebRequest request = (WebRequest)ar.AsyncState;
         int w2; int io2;
         ThreadPool.GetAvailableThreads(out w2, out io2);
         int cid2 = Thread.CurrentThread.ManagedThreadId;
         Console.WriteLine("End:" + w2.ToString() + ";" + io2.ToString() + "; id = " + cid2.ToString());
         var response = request.EndGetResponse(ar);
      }catch (Exception ex){ }
}

Result:
Begin:5;3; id = 10
End:5;2; id = 13
one io thread was used to execute the callback

2. test with BeginExecuteReader
Code:

public static void IoThread1()
{
     ThreadPool.SetMinThreads(5, 3);
     ThreadPool.SetMaxThreads(5, 3);
     int w; int io;
     ThreadPool.GetAvailableThreads(out w, out io);
     int cid = Thread.CurrentThread.ManagedThreadId;
     Console.WriteLine("Begin:" + w.ToString() + ";" + io.ToString() + "; id = " + cid.ToString());
     SqlConnection connection = new SqlConnection(connectionString);
     connection.Open();
     AsyncCallback da = new AsyncCallback(IoThreadCallBack);
     SqlCommand command = new SqlCommand(s_QueryDatabaseListScript, connection);
     IAsyncResult ir = command.BeginExecuteReader(da, command,System.Data.CommandBehavior.CloseConnection);
     ManualResetEvent waitHandle = new ManualResetEvent(false);
     waitHandle.WaitOne();
}
public static void IoThreadCallBack(IAsyncResult ar)
{
     int w; int io;
     ThreadPool.GetAvailableThreads(out w, out io);
     int cid = Thread.CurrentThread.ManagedThreadId;
     Console.WriteLine("End:" + w.ToString() + ";" + io.ToString() + "; id = " + cid.ToString());
     SqlCommand command = (SqlCommand)ar.AsyncState;
     StringBuilder sb = new StringBuilder();
     try
     {
           using (SqlDataReader reader = command.EndExecuteReader(ar))
           {
                while (reader.Read())
                {
                    sb.Append(reader.GetString(0)).Append("; ");
                }
           }
      }
      catch (Exception ex){ }
      finally
      {
           command.Connection.Close();
      }
}

Result:
Begin:5;3; id = 10
End:4;3; id = 7
another work thread was used to execute the callback
what's the problem....