Don't discount the fact that in the past we've seen VIN assignments to occur over a weeks, whilst the build run all occurred over the course of a couple of days. It's a slow process, I had my Model 3 assigned once it was already on the ship from Shanghai.
My guess is that it's a manual process (notice how nothing happens over the weekend or even outside of business hours?) trying to balance the delivery capacity maximum across the Tesla network with the different available configurations. Whatever it is, nobody so far has been able to decipher the logic behind it based on the limited data that's been crowd sourced.