Remember, up through 1968 or so, in many states, the vehicle was registered and titled for the year it was first SOLD, not necessarily the model year. If you bought a '66 Dodge on October 15th, 1965, it was registered and titled as a '65. If you bought a '66 Dodge that sat around on a lot and still on an MSO, that sold in February 1967, it was sold as a 1967 Dodge.
A good example of this is with Metropolitans. The last one to roll off the line was built in August 1960, as a 1960MY car. It was a pretty dated car by then, having first sold in 1954 through Nash and Hudson (and later, AMC) dealers. There was quite a stock of cars left in inventory nationwide, so the leftovers were sold as 1961 and 1962 Metropolitians. The last new Met was sold in early 1962.