I thought the same thing, thats what puzzled me. I was confused thinking why on earth I dont have to forward ports when hosting some newer games (like portal2 for example)? It just didnt make any sense to me how the router could "autoMAGICally" know what internal ip to send it to.
I thought they might have made it so the router somehow sends the packets to every computer behind the router or something crazy like that.
I'll take a look at the link and see if that helps explain things, thanks for the info!