Łukasz Kaiser basically confirmed it in a podcast:
https://youtu.be/3K-R4yVjJfU?si=JdVyYOlxUbEcvEEo&t=2624
Q: Are the releases aligned with pre-training efforts?
A: There used to be a time not that long ago, maybe half a year, distant past, where the models would align with RL runs or pretraining runs ... now the naming is by capability. GPT5 is a capable model; 5.1 is a more capable model