Maybe they use the same f/n for both the outbound and inbound flights, I know some do he round trip on the one f/n
Freighters often use the same flight number for different routes dependent on the day of the week, not easy. About the only constant is the initial departure point.
Busy night at Prestwick, 4 Afghan C208s just arrived with 3 Iraqi C30Js due in the early hours all on delivery.
Alan