How did "nine to five" come to mean a standard working day? Nobody I know actually works nine to five.

Nine to five wouldn't even be a full eight hours unless (a) you skipped lunch, or (b) your lunch break was considered part of your working day, and you were paid for it—neither of which is standard practice where I live (in Minnesota).

A standard day here is 8 a.m. to 4:30 p.m., with a half-hour break for lunch, for which you don't get paid. Thus you get paid for 8 hours.