Uneven quality on same models?

I have noticed very big difference in quality, while using the same model. I am wondering if this is an inference/compute time allocated problem? In particular, I noticed this on Fable and later Opus models. When i have seen Anthropic getting more GPUs, quality is generally better on output. But now, for instance, Fable quality was extremely poor last days. Even such as that it can't do prompt adherence, loose context memory very quickly and fail on tasks that clearly should be accomplished on single shot prompts. If its truly the same model, same context, similar prompts and context/memory, the output should be the same in quality if compute time is the same. Is there any way of checking/verifying this? Its quite difficult to work with LLM related projects if output quality differs so much as I have noticed.