[xx_ns]

Am I missing something here? Colonial merchant ledgers and 18th-century accounting practices have been extensively digitized and discussed in academic literature. The model has almost certainly seen examples where these calculations are broken down or explained. It could be interpolating from similar training examples rather than "reasoning."

[ceroxylon]

The author claims that they tried to avoid that: "[. . .] we had to choose them carefully and experiment to ensure that these documents were not already in the LLM training data (full disclosure: we can’t know for sure, but we took every reasonable precaution)."

[blharr]

Even if that specific document wasn't in the training data, there could be many similar documents from others at the time.