No doubt that is a part of it. Do you really think there is a significant difference between a (for example) 1974 Columbia 26 and a 1975 Columbia 26? Prior to a month or two ago, NADA was able to accomplish that feat. I know they went back to 1970, at least and am fairly confident they went back to 1968 which is the oldest year for my model.
There were also a lot of boats built in the 60s and 70s that are still chugging along. Yachtworld lists 562 sailboats under 51 feet for sale in the US and Canada built between 1970 and 1974. 1270 for 1970 to 1979. Definitely fewer boats in that era but I think there is something else going on here.
I'm guessing they did a 25 year whack of the data for bean counting purposes right around the decade mark. Which would make sense if they printed paper versions. But storage is practically free in the computer world.
Just a Columbia 28