I'm sure that each number has it's own significance; I believe
that 9 is the most frequently hit total (much as "3" in NFL)
and undoubtedly carries a large premium to buy on or off.
Also (my own speculation) it seems that even numbers are
more likely NOT to be hit because ties are resolved; in
other words, 4-4 is not the final score, so it seems to me that
it is less profitable to buy on or off 6, 8, 10 (?)