Having re-watched that movie recently, he's not wrong -- that's a deeply odd book for an apparent 8 year old girl to be holding. And with the amount of aliens that look like humans across the movies...
They call it entrapment - the officials put him in a position where be believes he's required to shoot in order to pass a test, but he sees no reason to. So finally he has to go with his gut and shoot the most probable target, even if he would have if not placed in that situation with those expectations.
