Different airlines have different policies as regards flight numbers. As said some use even outbound, odd inbound (or vice versa). Some do even for northbound and westbound, odd for eastbound and southbound, or any variation of that. All depends. Many airlines use alpha numeric callsigns with ATC so the flight number on your ticket is totally different from the aircraft callsign. E.G. Aer Lingus DUB-LAX is EI145 on the ticket, EIN1AK on the airwaves.
There's a few nerdy ones out there. The BA LHR-SEA flight is BAW5EA (5 looking like an "S"), so the flight ident looks like it's spelling SEA on the radar screen.
Yes I'm sad enough to notice these things when the aircraft pop up on my screen.