The 2019.15.x series is/was pretty limited in its reach. I don't know if it's for beta testing, cars with the HW3 Autopilot computer, or what, but it's for cars that are "special" in some way. If nobody else has more definitive information here, you might want to call Tesla to ask about it. Note, however, that Tesla software version numbers can be confusing. In particular, a lower number is not necessarily less feature-rich. In fact, some lower-numbered builds include features that aren't present in higher-numbered builds. Thus, the fact that you've got a lower-numbered build than what many other people have isn't necessarily an indication that what you've got is inferior. I don't happen to know what precisely is in the 2019.15.x series vs. the 2019.20.x that most other Model 3 owners have, though.
One more very minor point: Tesla has recently begun rolling out 2019.24.1. Among the cars tracked by TeslaFi, this release seems to be restricted to Model S and/or Model X; it's showing no Model 3 cars with 2019.24.1 so far. This could change fast, though. I've owned my Model 3 for barely over three months, so my experience is still limited; but based on that limited experience, I'd expect to see something in the 2019.24.x series start rolling out in a big way across all Tesla models (S, X, and 3; I don't know if the Roadster even gets software updates) within a week or so.
