I find this extremely surprising. I would have expected dict structures to have higher semantic context associated with them.

They did. The KV-Markdown is essentially a dict with ``` wrapper, and INI which is similar scored very high as well. The worst performers were index-based rows like CSV or Markdown tables. JSON is in the middle with high context and more syntactic noise and less clear record labels.

The odd ones to me are HTML which uses th and td to make indexed-based rows but did better than JSON somehow, and XML which is like JSON with even more syntactic noise placing better than INI. If I had to guess I'd say because vast amounts of the web were in the training set.