I've been listening to ATC a lot lately and have noticed many flight numbers which differ from those listed on timetables. These are some I can think of off the top of my head:
Timetable / ATC
BAW213 > BAW71G
BAW215 > BAW51G
GSM735 > GSM7GB
I admittedly do not know much about ATC and was wondering the reasoning behind the "odd" callsigns.