With public benchmarks we're trusting the labs not to cheat. And it's easy to "cheat" accidentally - they actually need to make a serious effort to not contaminate the training data.

And there's massive incentives for the labs to cheat in order to get the hype going around their launch and justify their massive investments in training. It doesn't have to be the CEO who's directing it. Can even be one/a few researchers who are responsible for a specific area of model performance and are under tremendous pressure to deliver.

It's supposed to leverage a "generate vs. critique" gap in skill level as a form of self-improvement. It's easier to judge how good food is vs. make it.

But here's the thing. When it comes to code review, you need to be effectively as skilled as the person who wrote it. There isn't really a gap.

And then the real clincher is this. LLMs naturally have a skill gap between their judgement and generation skills as is. The reason is that they have superhuman pattern matching and memorization ability. They can use their memorized patterns as a massive crutch for their actual reasoning skills, but they can't do the same for judgement calls in code review.

The self-preference is almost certainly coming from post-processing, or more likely because the model name is inserted into the system prompt.