任务/线/委托.beginInvoke with Thread.Sleep()

在下面的示例中，只有_DisplayUsingThreads(timesToDisplay)真正并行执行操作。而另外两个_DisplayUsingTasks和_displayusingdelegate一次只执行4个任务(在四核机器上)，然后等待一秒钟再执行4个任务。为什么?

public partial class Form1 : Form
        {
            public Form1()
            {
                InitializeComponent();
            }
            delegate void DisplayDelegate();
            DisplayDelegate myDisplaySleep;
            private void btnGo_Click(object sender, EventArgs e)
            {
              const int timesToDisplay = 50;
                //_DisplayUsingDelegates(timesToDisplay);
                //_DisplayUsingTasks(timesToDisplay);
                //_DisplayUsingThreads(timesToDisplay);
            }
            private void _DisplayUsingTasks(int displayNumber)
            {
                for (int i = 0; i < displayNumber; i++)
                {
                    Task task = new Task(DisplayIdSleep);
                    task.Start();
                }
            }
            private void _DisplayUsingThreads(int displayNumber)
            {
                for (int i = 0; i < displayNumber; i++)
                {
                    Thread thread = new Thread(DisplayIdSleep);
                    thread.Start();
                }
            }
            private void _DisplayUsingDelegates(int displayNumber)
            {
                myDisplaySleep = DisplayIdSleep;
                for (int i = 0; i < displayNumber; i++)
                {
                    myDisplaySleep.BeginInvoke(null, null);
                }
            }
            private void DisplayIdSleep()
            {
                Debug.WriteLine("Thread Id : {0}", Thread.CurrentThread.ManagedThreadId);
                Thread.Sleep(1000);
            }
        }