Let me try to explain: On routes with high frequency flights, airlines will generally issue a different flight number for each unique departure time to a particular destination. E.g. YY500 departing at 9am to LAX on Mon/Wed/Fri and YY502 departing at 9:05am to LAX on Tue/Thu/Sat/Sun. It's unusual to have the same flight number YY500 apply to either the 9am or 9:05am flight. But if there is no collision (i.e. 9am and 9:05 doesn't operate on the same days) it seems a sensible optimization to reuse the flight numbers. It's just not very common practice in the airlines I regularly fly with (NZ, QF, JQ etc) so can understand why it seems remarkable. Air New Zealand for example only 'reuses' flight numbers for their flagship (e.g. NZ1, NZ2, NZ3, etc) flights whereas their NZ5xx NZ6xx series flights for high frequency domestic flights generally are unique in respect to departure times and rarely do they reuse a flight number for a different departure time.

Flight # of Theseus?

Like I said, 1 day of separation doesn't seem like anything remarkable, but it begs the question: do the numbers get reused for the same flight on shorter intervals than a day? If so, when? Even more interesting, why?

And, technically, the two flights 1555 in my example aren't the same - one departed at 4:00, and the other at 4:20. Both flights were on-time, so this wasn't a scheduling issue delaying one or the other. 20 minutes isn't a big difference, but it is still a difference. How big does the difference have to be to make Southwest assign two different codes?

Even more philosophically asinine, what do we even mean when we say two flights are "the same?" If Southwest thinks those two flights are the same, then clearly departure time only matters within a certain fudge factor. So that's one part of the picture. Do the make and model of plane need to be the same? Does it need to be the very same physical airplane?

Are any of these questions important to my everyday life? No! Are they interesting? Absolutely!