To answer Premex2000's question,

By using a single flight # for an itinerary, the flight will show up on the first page of a Travel Agents Reservation screen.

On routes that have no competing N/S service, this gives the carrier an advantage over carriers whose choice displays as a connection.

This practice shows the importance of knowing the difference between Non-Stop and Direct. A Non-Stop is just that, Point A to Point B with NO stops. A direct flight will get you from point A to point B, but you will stop and might have to change planes.

Sadly, most travellers think there is no difference between Direct and Non-Stop.

This lack of understanding is what gives the change of guage flights an advantage. A customer could call a T/A and ask for a flight from San Antonio to Tokyo. The T/A will then tell the customer "CO has a Direct Flight". Customer purchases ticket thinking "Non-Stop" and probably does not even think to question this when they board an MD-80 in San Antonio.