Other mistake: Norman walks out of his mother's house with an umbrella in his hand in the pouring rain and yet is bone dry when he reaches the motel. The woman as well, despite going back and forth from her car in the pouring rain, remains dry.
Other mistake: When Marion Crane trades cars she drives off the used car lot with California license plates on her newly purchased vehicle. The dealer should have transferred her old Arizona license plates to the new car.
Other mistake: In the scene where Bates leaves her motel room to back her car up to the door, he doesn't use keys to start the car.
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