Yes, I've noticed this too. Each pixel in actuallity is wider than taller. So no matter how it looks on your computer monitor it will always look a little squished on the AVIC unit. Maybe physically measure the screens dimmensions in mm to get a more accurate aspect ratio.
Well if you really want to break it down. Since the screen resolution is 800x480 this would make it exactly 5:3 (which is the same as15:9 if you wanna compare it to 16:9)