I've wondered for a long time why they use 39 points instead of 24 points or even 30 points which seem to make more sense to me.

By the way, so far in two rounds of our AFL tipping comp, it's 10-6 in favour of the 1-24 margin. Had we been using 1-39 points, it would have been 13-3. So you see it just makes it a bit more even.