Where does TForm maximized Left Offset of -8 comes from

Question

When maximizing a plain TForm via Form.WindowState := wsMaximized; on the main screen, the property Left is -8 and Width is e.g. Screen.Monitors[0].BoundsRect.Width+8.

When i want to set the Forms absolute position via 'Form.Top, .Left, .Width, .Height' to the active Monitors WorkingArea, there is always a gap due to the missing 8 pixels.

Where is the additional offset of 8 in each direction coming from and why is it necessary?

My test form (with a button to toggle some positioning states)

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    procedure Button1Click(Sender: TObject);
  private
    { Private-Deklarationen }
    FState: Integer;
  public
    { Public-Deklarationen }
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  LBRect,
  LWRect,
  LFCRect,
  LFBRect: TRect;
begin
  LBRect  := Screen.Monitors[0].BoundsRect;
  LWRect  := Screen.Monitors[0].WorkareaRect;
  LFCRect := Self.ClientRect;
  LFBRect := Self.BoundsRect;

  case FState of
    0:
    begin
      Top := 100;
      Left := 100;
      Width := 800;
      Height := 600;
    end;
    1:
    begin
      WindowState := wsMaximized;
    end;
    2:
    begin
      WindowState := wsNormal;
    end;
    3:
    begin
      Self.Top := LBRect.Top;
      Self.Left := LBRect.Left;
      Self.Width := LBRect.Width;
      Self.Height := LBRect.Height;
    end;
    4:
    begin
      Self.Top := LWRect.Top;
      Self.Left := LWRect.Left;
      Self.Width := LWRect.Width;
      Self.Height := LWRect.Height;
    end;
  end;

  Inc(FState);
  FState := FState mod 5;
end;

end.

When setting the state to 3 or 4, there will be a gap. Why?