I know on AS, north and westbound flights have odd numbers, and south/eastbound flights have even numbers. They're also grouped pretty closely by route, e.g. single-digit flights (1-6) are to/from DCA, 10s and 20s are BOS, 30s are MSP, 60s are Southeast AK milk runs, 80-120 or so are ANC-SEA, 450-480s are LAX, 610s are LAS, 730s are IAH, 800s are Hawaii, etc.

I may be wrong, but I think the OP is looking for some sort of similar rhyme or reason for WN flight number assignments.