My guess is there is a delay at the start of a unit as it begins production. Supplies for that particular unit are gathered. A frame is built and polished. Tires and suspension put on. And then four main units produced. My guess is a production number is assigned well before a unit actually is a rolling chassis.
I had heard that vin #/hull numbers are assigned earlier now than before. I knew when we picked our unit up we had to call back home with our number to get our insurance activated. Tenn. temp license was good for 30 days, but it would have been nice to register at home first, as we had expired plate for about two weeks on our leisurely trip home. I think they changed when some folks needed the numbers for financial reasons...