Another oddity of BA's flight numbers is that on lower numbered flights the outbound is odd, and the inbound is that +1, but on flights above about 50, the inbound is the outbound's number -1, eg BA 5/6 to HND, BA 27/28 to HKG, but then BA 59/58 to CPT and BA 287/286 to SFO. Is there a historical reason for this?