I have no idea why flight numbers would have anything to do with crew scheduling. Crew can be scheduled to do a same plane turn regardless of the flight number, and vice versa. It may well be true that a given route with a same flight number turn is also typically scheduled to have the same crew, but I see no reason why crew scheduling and flight numbers would be tied to each other at all. Is it actually the case that they’re tied to each other (rather than happen to be the same) in any real case?