Probably for the same reason I said above -- conflicting flight number.
The stub origination example I gave would have the same company flight number as the stub outbound and delayed inbound operation to the same airport.
It also applies at the destination airport when someone like Jetbloo or Duhlta has a flight 2110 in or out of JFK around the the same timeframe.
I forget exactly what the radio call suffix scheme was, but it was something like flights with one to three digits get a P suffix, flights in the 1000 range drop the 1 and add a Q, and flights in the 2000 range drop the 2 and add the R... or something like that.... it's been years since I worked a radio in ops.
