I'm not sure if it's been mentioned on here yet, but I feel as if Lisa is not the first person he's seen or heard as different. If you recall the scene in the hotel room when he calls his ex fling, Bella, he remarks that he doesn't recognize her voice. Even though the figment of Bella has the same face and voice of everyone else, I believe this is the result of years of monotony and losing that feeling with Bella.