In delegate function, add the same listener event, why the same listener be called Immediately?

I write one script to listener an Event and in delegate function, i add another script the same listener in previous Event, but why the same listener be called immediately? The following is add another script the same listener script: using System.Collections.Generic; using UnityEngine;

 public class SceneManager : MonoBehaviour
 {
     private static SceneManager m_Instance;
 
     public static SceneManager Instance
     {
         get
         {
             if (m_Instance == null)
             {
                 m_Instance = (SceneManager)FindObjectOfType(typeof(SceneManager));
             }
             if (!m_Instance)
             {
                 Debug.LogError("SceneManager could not find himself!");
             }
             return m_Instance;
         }
     }
 
     [HideInInspector]
     public List<EventDelegate> EventDelegetDo = new List<EventDelegate>();
 
     public void RegisterBackBtnDo()
     {
         EventDelegate.Add(EventDelegetDo, Do);
     }
 
     public void UnRegisterBackBtnDo()
     {
         EventDelegate.Remove(EventDelegetDo, Do);
     }
 
     private void Start()
     {
     }
 
     private void Update()
     {
         if (Input.GetKeyDown(KeyCode.T))
         {
             EventDelegate.Execute(EventDelegetDo);
         }
     }
 
     private void Do()
     {
         Debug.Log("===============ZJDebug=============== From SceneManager");
     }
 }

The following is initial register listener script:

 using UnityEngine;
 
 public class TDelegate : MonoBehaviour
 {
     public void RegisterBackBtnDo()
     {
         EventDelegate.Add(SceneManager.Instance.EventDelegetDo, Do);
     }
 
     public void UnRegisterBackBtnDo()
     {
         EventDelegate.Remove(SceneManager.Instance.EventDelegetDo, Do);
     }
 
     private void Start()
     {
         RegisterBackBtnDo();
     }
 
     private void Update()
     {
     }
 
     private void Do()
     {
         Debug.Log("===============ZJDebug=============== From TDelegate");
         SceneManager.Instance.RegisterBackBtnDo();
         //Invoke("WaitForTest", float.NegativeInfinity);
     }
 
     private void WaitForTest()
     {
         SceneManager.Instance.RegisterBackBtnDo();
     }
 }

I have tried multiple variations of this, if invoke float.NegativeInfinity second register it not be called Immediately, can someone tell me why? in addition to delay call There is no better way called by after? English is too bad please forgive me thanks!