A lot of things are ambiguous in the film. After Arthur goes to talk to Thomas Wayne and gets punched, Arthur leans over the bathroom sink in the theater, then the film cuts to Arthur leaning over the kitchen sink in his apartment shirtless. After that he goes into the fridge. So was he even in the theater talking to Wayne or was it all in his head? At the end of the film when he's talking to the doctor, he could be recalling what happened and telling the therapist the story we just saw from his point of view, so some of the events happened, but not necessarily they way he remembers them, since he's not a reliable narrator. He also looked different in the last scene. His face looked fuller.