Err - that is except for the odd numbered ones that go west and also except for the even numbered ones that go east.
AFAIK flight numbers are entirely company specific and company assigned, but do follow some conventions within companies. Perhaps that is the case with the east/west thing in Junk's company.
Also some marketing involvement - a lot of flights to and from HKG for instance have one or more 8s in their number (8 is seen as a lucky in that part of the world).
Hope that helps.
Esrev.