The numbering system might be based on railway practice, where the determining factor is whether the train is travelling away from, or toward, the country or provincial capital city. From my own somewhat limited observation of flight numbering, it seems that odd numbers are used for flights away from the main base and even numbers used for flights to the main base. This might change a bit when a company has several bases.
As to the allocation of numbers, I'm sure that it's company-specific, perhaps with the lowest numbers used on flights to the operator's most significant destinations. And, as has already been said, it's possible that operators include an "8" in flight numbers for those flights which go to South East Asia.