Also, some airlines will keep a general range for flights serving a particular region. For example, most (all?) UA flights to/from Asia are 8xx. All NW flights to/from Asia are single digit or low-2-digit numbers. (Not sure where the transition occurs to to/from AMS flights because SFO-NRT is 27/28 and I'm sure there's a 3x between DTW and AMS)
More on the same: NW SJC-MSP are 19x, SFO-MSP are in the 35x-36x range etc etc.