Overall can be calculated, but going back over the last two GameDev.tv jams, it looks like you got to vote for Overall. In the Halloween jam I had 70 people vote on my game and it brought my ranking down somewhat due to law of averages. (I came in 12th overall.) I literally watched it go lower in the overall rankings the last two days. But the game that came in second also had 70 votes. Two other games that beat me had 45 rankings, and all the others had 15 or less.
So it really depends on what the people who rate your game think of it. All-in-all jams, like any judged competition are subjective. Different people like different things. All you can do is do your best.