amazing, huh? although it poses a problem if the car isn't introduced in January (like they usually are in those parts of the world). You'll have two different types of 2009 models which may confuse used car buyers. The DMV doesn't care either. They go by plates so if your car was registered in 2009 with a 2009 plate, it's a 2009 car.
For many years (and it might still be true) Mercedes didn't really have year models. For the US market they were called by year, but to buy parts you needed the chassis serial number and the engine serial number. They made running changes when they felt like it, and the only real way to know which part, in some cases, was by serial number. Another interesting case was the Alfa Romeo Sprint Speciale. All of the bodies were built in one year (1959, I believe) and the cars were assembled and sold through about 1965. So the newer ones had as old a body/chassis as the older ones (worse, since they sat outside in racks and tended to pre-rust before assembly).
Now I understand why my British friend is always giving me a hard time about "you Americans and your years" ...I never understood what she was saying until now. Our way of doing it makes perfect sense to me though. Models just come out a little bit before the start of the new year and usually in limited numbers. The 2010 Hyundai Genesis coupe was an exception though. That thing was on showroom floors back in March of this year!