I get the tokenization argument and it may influence it a bit, but I suspect the n-digit math issue has to do more with search the way it samples (in the bpe link gwern references some experiements I'd done with improving n-digit math by chunking using commas, http://gptprompts.wikidot.com/logic:math). I think since it samples left to right on the first pass, it's not able to predict well if things carry from right to left.
I think can mitigate the search issue a bit if you have the prompt double-check itself after the fact (e.g. https://towardsdatascience.com/1-1-3-wait-no-1-1-2-how-to-ha...). Works different depending on the size of the model tho.