NADA is based on actual transaction values reported by DMV agencies around the country. Given the small number of transactions, it is very possible that the distribution of values for the '73 year is possitively skewed, resulting in the arithmatic mean being larger for the '73 year than for the '72 year because of 1 or 2 speicific transactions. Of course, the '72 year values could be negatively skewed. With small sample sizes, a single transaction can have a huge effect on the mean.
There are classic car pricing guides out there which are not solely based on transaction values and which use the 5-point rating system for vehicle condition. Still, the problem here is simply a problem of small numbers.