I think both Gene's and Chipper's responses both have merit. May I elaborate some?
After reading what they said, I nonetheless, I think maybe owing to the factory experience from 1911 or so on for a few years, they built what they thought they could best sell any given year. I even bought a Chevette (sp). Once. Gas crunch. Need I say more?
It was and is somewhat of a gamble when introducing new styles, as Gene and Chipper pointed out, but over time that and the dealer inventory of what was left over unsold from one production year to the next pretty much dictated what they built. Then and now.
And for popular models what a dealer could get. The dealer in Lake View SC told me years ago that he was allotted one (1) Corvette per year. It was a small dealership. He told me that each year it was sold even before he got it.
Given the above and what Gene and Chipper said, I think those factors helped the factory estimate what they should produce any given production each year.
The only puzzlement come in when one wonders why in the world did they produce so many for 1937 and 1938. Who would want one of those... er,... beauties? You can chalk the sells of those two up to marque loyalty and salesmanship.

(Juuussstttt kidding)
Now for the 1941 models they couldn't produce enough. Those eye appealing works of art sold like hot cakes. Did too!
Interesting question.
Charlie
