It seems plausible to me that RL improvements allowed Anthropic to improve on Opus 4.8, similar to how OpenAI substantially improved upon GPT 5.5 with 5.6 Sol.
Fable 5.1 and GPT-6 are rumored to launch in August, presumably bringing those improvements to the larger models.
I don't know how systematic Anthropic are about their versioning - I'd have guessed that major version number increases (4.x -> 5.x) reflect different base models (different pre-training runs), in which case Opus 5 would be a distilled version of the Fable 5 base model (but without the cyber exploit post-training), rather than being Opus 4.8 with additional post-training, but who knows? I don't believe Anthropic have said anything about this.
OpenAI's versioning seems equally opaque. Claude mentioned that there was a tiny bit of clarity from them in that GPT 5.5 was the result of a new pre-training run, which would seem to strongly suggest that 5.6 (following so soon after, and given the choice of naming) is therefore based on 5.5 - but again who knows.
It seems that so much of the model performance is now coming from post-training that this is what is driving inter-version performance differences, and that base models are much less important than they used to be.
I think the best proxy for this feeling is the Artificial Analysis' omniscience index. Fable has a 40 score, and Opus (4.8) has 27.