My guess is they just trained gpt3.5-turbo-instruct on a lot of chess, much more than is in e.g. CommonCrawl, in order to boost it on that task. Then they didn't do this for other models.
People are alleging that OpenAI is calling out to a chess engine, but seem to be not considering this less scandalous possibility.
Of course, to the extent people are touting chess performance as evidence of general reasoning capabilities, OpenAI taking costly actions to boost specifically chess performance and not being transparent about it is still frustrating and, imo, dishonest.
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