Flip side is that users who comment a lot on popular stories but don't post much tend to get overcounted. My karma/post is like 25-30, but only because I get the majority of my karma from comments. My posts themselves average like 5 points.
I've picked up literally 100 karma in the last week or so, yet made only one submission in the last month. A couple popular comments can heavily distort the averages.
Sorry about the confusion: by post I meant either news stories or comments, no distinction.
We could use the median rather than the mean, but I see no problem in the fact that a few very popular comments can pump up your average. They boost your absolute count too. And that's OK. Karma measures popularity.