Flakern trotz DoubleBuffer?

[xZise]

Bei mit Flackert eine ProgressBar und ein TLabel, trotz
DoubleBuffered := true ! Habe ich da einen Fehler gemacht?

Delphi-Quellcode:

procedure TForm1.Button2Click(Sender: TObject);
var
  i, j, Start, l : Integer;
  k : Extended;
  p: ^PixArray;
begin
  Start := GetTickCount;

  PBProgress.Max := Image.Height * Image.Width * 3;
  PBProgress.Position := 0;

  k := 255 / (TBColors.Position - 1);

  for i := 0 to Image.Height - 1 do begin

    p := Image.Picture.Bitmap.ScanLine[i];

    for j := 0 to Image.Width do begin

      for l := 1 to 3 do begin
        p^[l] := Round(Round(p^[l] / k) * k);

        PBProgress.Position := PBProgress.Position + 1;
        PBProgress.Refresh;
      end;


      LTime.Caption := 'Working (' + IntToStr((GetTickCount - Start) div 1000) + ' s)';
      LTime.Refresh;

      inc(p);
    end;
    if CBRefresh.Checked then
      Image.Repaint;
  end;

  LTime.Caption := 'Ready (' + FloatToStr((GetTickCount - Start) / 1000) + ' s)';
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
  DoubleBuffered := true;
end;