Or he could be a night owl, or live in a timezone that doesn't match prime upvoting hours.

Interesting, I never considered that voting would be so sensitive to these sorts of variables. Although this could have some effect, I'll be the first to admit my ability to communicate through this medium needs some serious work.

I don't see how a comment's upvote-worthiness changes with time of day.

> comment's upvote-worthiness changes with time of day

You have fewer people around to upvote you.

Another issue is that if you read HN through the RSS feed, you will almost always be commenting on older submissions.

Perhaps. My views, interests, and experiences invariably differ from the status quo around here. I'd like to think that has little effect though--my karma per post is within range of the top contributors'(http://news.ycombinator.com/leaders)*

*To be fair, the list is quite sporadic. Min = 1.2, Max = 19.61, Avg = 4.909.