Yuck. I hope it's a rolling average (i.e. the average from the last 30 days). My account is 1368 days old! How the heck am I supposed to change my average in any significant way?
The average is calculated by taking your most recent 50 comments, dropping the first five, dropping the max, and averaging the rest. Yours is currently 7.14 (it's in your profile), a very high average. For comparison, of the 100 people on the leaderboard, there are only nine with a higher average.
Slightly off topic, but do you have any idea how often the average is recalculated? I only ask because I've noticed that my average hasn't changed since it appeared in the top bar, yet I've doubled my total karma in the last two weeks.
Based on the formula you just gave, my average should be 3.27, but it still shows 2.55.
I realize it's a bit of a meaningless metric, but it's something I've been idly pondering for a few days.
"There's a background process that runs every 45 seconds and picks a random loaded user to update the average of. Empirically, not more than every couple days, at best."
Ah, good. Arc 3.1 was released sometime before 2009-08-04, since those are the timestamps of my unpacked .jar file. The post you linked is about six months old.