Why does CM_CONTROLLISTCHANGE performs for indirect parent controls?

I noticed that if I have a main container/parent (MainPanel), adding a child panel to it (ChildPanel), will perform CM_CONTROLLISTCHANGE on MainPanel (in TWinControl.InsertControl()) which is fine.

But if I insert a child control (ChildButton) to ChildPanel, a CM_CONTROLLISTCHANGE will be fired again for the main MainPanel!

Why is that? I was expecting the CM_CONTROLLISTCHANGE to fire only for ChildPanel when inserting ChildButton into ChildPanel.

MCVE

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls, StdCtrls;

type
  TMainPanel = class(ExtCtrls.TCustomPanel)
  private
    procedure CMControlListChange(var Message: TCMControlListChange); message CM_CONTROLLISTCHANGE;
  end;

  TForm1 = class(TForm)
    Button1: TButton;
    Memo1: TMemo;
    procedure Button1Click(Sender: TObject);
  private
  public
    MainPanel: TMainPanel;
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TMainPanel.CMControlListChange(var Message: TCMControlListChange);
begin
  if Message.Inserting then
  begin
    Form1.Memo1.Lines.Add('TMainPanel.CMControlListChange: Inserting ' + Message.Control.ClassName);
    // Parent is always nil
    if Message.Control.Parent = nil then Form1.Memo1.Lines.Add('*** Parent=nil');
  end;
  inherited;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  ChildPanel: TPanel;
  ChildButton: TButton;
begin
  FreeAndNil(MainPanel);

  MainPanel := TMainPanel.Create(Self);
  MainPanel.SetBounds(0, 0, 200, 200);
  MainPanel.Parent := Self;

  ChildPanel := TPanel.Create(Self);
  ChildPanel.Parent := MainPanel;

  ChildButton := TButton.Create(Self);
  ChildButton.Parent := ChildPanel; // Why do I get CM_CONTROLLISTCHANGE in "MainPanel"?
end;

end.

DFM

object Form1: TForm1
  Left = 192
  Top = 114
  Width = 685
  Height = 275
  Caption = 'Form1'
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'MS Shell Dlg 2'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Button1: TButton
    Left = 592
    Top = 8
    Width = 75
    Height = 25
    Caption = 'Button1'
    TabOrder = 0
    OnClick = Button1Click
  end
  object Memo1: TMemo
    Left = 456
    Top = 40
    Width = 209
    Height = 193
    TabOrder = 1
  end
end

P.S: I don't know if it matters, but I'm on Delphi 5.