"Who says he's seeing this place the way we're seeing it? Humans see what they want to see. You're very stubborn—er, persistent, that way?"
Even when they get to the Underworld, Percy still seems to have a hard time understanding that humans understand things in different ways. Grover calls Percy "stubborn," but we have to remember that Percy has only known that he is a half-blood for two weeks. Cut the guy some slack!