What did the end of American Psycho mean?

Yet another question for a Bale movie. I don't know if my mind is not working or something, I'm just not getting the ends of his movies. First with The machinist, and now with AP. So...I didn't get where everything he did didn't actually happen. The murders, Paul Allen's house, the prostitutes, and such. Even the phone call to his "lawyer", the guy acted like it was a joke and like Bateman was another person. And yet it shows his istant in his office with his date book, looking at all the drawings. What the hell?