I sometimes wondered if the figures just happened to be numbered as part of the standard production process
Nah, as I said, male Starfleet legs were all numbered at once, so there wasn't even a guarantee that every figure had every number represented.
to try and boost the "collectibility".
It was exactly that. It certainly did excite a section of the collectors, and that's all you need to create word-of-mouth publicity. Except when it backfires, like the 1701s.