This (and the price increase) points to a new pretrained model under-the-hood.
GPT-5.1, in contrast, was allegedly using the same pretraining as GPT-4o.
I don't think it really matters what's under the hood. People expect model "versions" to be indexed on performance.
OpenAI might have learned not to overhype. They already shipped GPT-5 - which was only an incremental upgrade over o3, and was received poorly, with this being a part of the reason why.
My hunch is that this is the same 5.1 post-training on a new pretrained base.
Likely rushed out the door faster than they initially expected/planned.
It was a generational leap if there ever has been one. Much bigger than 3.5 to 4.