I've seen a few routers where the upnp sometimes gets itself in a bit of a twist and then fails to map ports as required.
Usually a reboot of the router sorts that little problem.
Never did the Nat test on live recently but I wouldn't be surprised if I sometimes get strict Nat since we have 2 xbox 360's depends if they both try and use the same ports at the same time.
Saying that I've had 3 online before playing halo3 and not had a problem