Why does the class name of the TForm descendant matter?

Question

In the VCL Forms application below, the Application.MainForm is hidden and another TForm descendant is shown instead. The strange thing is that only if the name of this TForm descendant starts with "TForm", it can be seen when Alt+Tab is pressed. If its name does not start with "TForm", it can not be seen when Alt+Tab. The above is tested with Delphi XE-Berlin on Windows 10 Ann Edition x64.

Could you help to comment the reason why the class name matters here ? O_O

More over, does the class name of the TForm descendant matters somewhere else ?

program

program Strange;

uses
  Forms,
  uHiddenMainForm in 'uHiddenMainForm.pas' {HiddenMainForm},
  uActualMainForm in 'uActualMainForm.pas' {FormActualMainForm};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(THiddenMainForm, HiddenMainForm);     // Note: WRONG ! Still, interesting strangeness.
  Application.ShowMainForm := False;
  Application.MainFormOnTaskbar := True;
  // Application.CreateForm(THiddenMainForm, HiddenMainForm);  // Note: CORRECT
  with TFormActualMainForm.Create(HiddenMainForm) do
  begin
    Show;
    Update;
  end;
  Application.Run;
end.

unit for the main form (i.e., Application.MainForm)

    unit uHiddenMainForm;

    interface

    uses
      Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs;

    type
      THiddenMainForm = class(TForm)
      end;

    var
      HiddenMainForm: THiddenMainForm;

    implementation

    {$R *.dfm}

    end.

unit for the actual "main" form

    unit uActualMainForm;

    interface

    uses
      Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs;

    type
      TFormActualMainForm = class(TForm)
        procedure FormClose(Sender: TObject; var Action: TCloseAction);
      protected
        procedure CreateParams(var Params: TCreateParams) ; override;
      end;

    implementation

    {$R *.dfm}

    { TFormActualMainForm }

    procedure TFormActualMainForm.CreateParams(var Params: TCreateParams);
    begin
      inherited;
      Params.ExStyle := Params.ExStyle or WS_EX_APPWINDOW;
    end;

    procedure TFormActualMainForm.FormClose(Sender: TObject; var Action: TCloseAction);
    begin
      Application.Terminate;
    end;

    end.

The problem If its name does not start with "TForm", it can not be seen when Alt+Tab. shows itself with the following code. The only difference is the change of class name from TFormActualMainForm to TXFormActualMainForm.

program

program Strange;

uses
  Forms,
  uHiddenMainForm in 'uHiddenMainForm.pas' {HiddenMainForm},
  uActualMainForm in 'uActualMainForm.pas' {XFormActualMainForm};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(THiddenMainForm, HiddenMainForm);     
  Application.ShowMainForm := False;
  Application.MainFormOnTaskbar := True;
  with TXFormActualMainForm.Create(HiddenMainForm) do
  begin
    Visible := True;
  end;
  Application.Run;
end.

unit for the actual "main" form

    unit uActualMainForm;

    interface

    uses
      Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs;

    type
      TXFormActualMainForm = class(TForm)
        procedure FormClose(Sender: TObject; var Action: TCloseAction);
      protected
        procedure CreateParams(var Params: TCreateParams) ; override;
      end;

    implementation

    {$R *.dfm}

    { TXFormActualMainForm }

    procedure TXFormActualMainForm.CreateParams(var Params: TCreateParams);
    begin
      inherited;
      Params.ExStyle := Params.ExStyle or WS_EX_APPWINDOW;
    end;

    procedure TXFormActualMainForm.FormClose(Sender: TObject; var Action: TCloseAction);
    begin
      Application.Terminate;
    end;

    end.