There are quite a few things to consider when assigning flight numbers. For instance, another carrier might have a similar flight number at the same time period. A lot of airlines us "711" for LAS flights, and you can't have two airlines operating with the same flight number in the same airspace. Also, it messes things up if you try to have one flight number operate through the same city pairs during one routing. ATC computers don't know which flight is which, and it would also cause confusion when booking a reservation. Our Schedulers like Bill Owen look at the lines of flight which have the aircraft routings, the flight numbers are filled in later.