The final exam was not take-home, which is where the massive discrepancy showed up.

I do agree that the idea of giving take-home exams and expecting students not to cheat has long since passed its prime. There may have been a time when it was reasonable to expect most students to behave honestly, but that does not hold at all in today’s climate. Especially post-COVID, for whatever combination of reasons, students just don’t seem to care at all about anything other than min-maxing their effort to grade ratio.

Which leads to a lot of outcomes like this one where students start using ChatGPT early, think it will continue forever, and then get completely crushed when they encounter the first assignment where they can’t cheat.