I see – my initial interpretation of OP’s “special case” was “Theory 2: GPT-3.5-instruct was trained on more chess games.” But I guess it’s also a possibility

I see – my initial interpretation of OP’s “special case” was “Theory 2: GPT-3.5-instruct was trained on more chess games.” But I guess it’s also a possibility that they had a real chess engine hiding in there.