man, people have such difficulty separating their semi-omniscient view from a characters perspective. Yes we know it is Arya, but that doesn't mean that it is obvious to Tywin. Yes we are reasonably sure that it isn't the Stark boys, but that doesn't mean that it is obvious to Luwin. He is a fictional character who is not aware that their arcs have not been completed and that from a story telling perspective they pretty much can't die yet.

cliffs: he think's those are the Stark boys, he is genuinely hysterical.