zeromemory string state question

[Madammar]

why some times when i do zeromemory for long string it does not get cleared from memory 

i do something like some times when i click on button 4 or 5 times i can see this text in memory viewer

Code:

procedure TForm1.OnClick(Sender: TObject);
var
 pstr: string;
begin
 pstr := 'abcdefghijklmnobqrsputwjdgdfkljfhghjfggffhgfj;

 ZeroMemory(@(pstr[1]), Length(pstr)*SizeOf(Char));

 pstr:='';

end;