My first assumption is that almost everything you post is seen as (at least somewhat) valuable (for almost every post #upvotes > #downvotes), so the net karma you get is mostly based on throughput. More readers, more votes. More votes, more karma. Second, useful posts do not only take time to write, they take time to read as well. And my guess is that most of us don't like to vote on thoughtful articles before we have read them. So for funny posts we can quickly make the judgement on how to vote, but for longer posts it takes time. Decision fatigue may also play a role (after studying something complex the extra decision of whether to vote on it feels like work so we skip it). People may also print more valuable texts, or save them for later, making it easy to forget to vote. The effect is much more evident on other karma based sites. Snarky one-liners and obvious puns are karma magnets. LessWrong uses the same system and is visited by the same species and therefore suffers from the same problems, just to a lesser extent.

I guess that's a little better. (also that sounds like poor accuracy rather than poor calibration, but that's probably just semantics).