Trivia: Director Gus van Sant has a cameo at the beginning of the movie when Marion enters the office after her lunch break - he is talking to someone looking just like Alfred Hitchcock.
Trivia: When Arbogast enters Norman's house and scans his surroundings there is a statue of an angel casting a shadow that resembles someone stabbing.
Answer: What is shown is what you would see in the rearview mirror. Being in the mirror, it's a reverse image. Think about if you were looking at the cars pulled over head on/facing the driver, the road and the driver would be on your right. Shots in the mirror though shows the road and the driver on your left, the reverse.
Bishop73