Because there's only one playlist window, that's what happens. That's a shortfall of the LW playlist function.

As far as opening an older saved playlist & "then" adding new songs to that, I'm not sure, it should work ok but I can't remember trying it. So I guess that's what you should try. Opening the older playlist & then adding songs to it. Otherwise if you create a new playlist, but then open an older one without saving the new one, then the new one will be lost.

As I said earlier, I'll check it out but not right now. Tomorrow.