OK, I didn't think to disable switch to thinking (didn't know this was a mode). When I did that then it did get it wrong -- oddly it took about the same amount of time, so thinking mode wasn't taking longer, but it was more accurate.

Right, though I didn't explicitly disable thinking for my first attempt either. I'd guess my prompt was less detailed than yours and so ChatGPT (in "Auto" mode) decided to allocate thinking tokens for your questions but not mine.