I can confirm I received an update within 48 hours of taking ownership. It was also only one update out from new. Busts the myth that new cars have “old stable software” and that software upgrades are delayed.
I think it's largely luck of the draw.
Tesla's policy of continuous improvement means that it's commonplace for new cars to ship with a hardware configuration that's slightly different from the existing cars in the field. Probably even more so since the chip shortage began, as Tesla has to constantly redesign things to use different parts.
When they introduce new hardware, they will need to introduce support for the hardware into the software builds. If it helps, think of it as including the right drivers for the hardware, although in reality it will probably be a bit more complicated than that, and will have implications for their software testing process, too. In order to get the cars out of the door quickly, they will probably cut a hardware support branch, which allows them to make a special release for that new batch of cars before the normal software works on them. However, it takes a bit of time for this new hardware support to get included in the normal releases. It may only go into trunk, so you may need to wait until a new release branch is cut - i.e. the week number (second component) of the software version is later than the date of the change. Maybe they sometimes backport the support to a recent release branch, but probably they usually don't. Hence there will be a period of time when the cars are on a special release, specific to that batch. Eventually, the software release processes catch up, and that batch of cars can run the normal software.
All just supposition on my part, but fits the observed facts, I think.
TL;DR I think it largely depends on how soon after a hardware change it is when you take delivery