First, you're asking the wrong people...anyone answering here will mostly be speculation.
That said, historically the cars go out in some kind of combinations, so like LR tend to go out to one area, then P out to an area, then specific colors go out at once, then...But in this case, a lot of cars are sitting on lots just seemingly waiting for the firmware updates to hit them, and they don't actually have the ability to push to specific cars in any kind of order that I've ever seen, so they're probably just loading up batches of cars for updates (like anything made in the last month or something) and the cars will get the updates when they do.
That means there could be newer cars getting the updates and being ready first, but they go to the people first on the list waiting for that combination of features in that area. If it happens that only 1 person in the Denver area needs that combination, for example, they'd be assigned the VIN since the car's already there at the delivery center, even if the only ordered 3 days ago. That's kind-of why the most common combinations are going in way more order than the stranger ones...because there's probably a lot more people waiting on those in the areas the cars are sitting than others.