Some airlines started their numberng system on 100 (for the big and best flight) returning as 101. Other started as flight number one (e.g. Clipper one). Again others use a A or B (depending on outbound or inbound), and again othes look at the flight of the day so your outbound may be 4357, and the inbound 5437. There are no rules, as you could also (theoretically) use the aircraft's registration as flight number. It would just make it so darn complicated for the marketing folks...
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There's nothing like a three-holer...