[TeMPOraL]

Maybe it's just growth? The more active users HN has, the more votes a popular comment will get. So active commenters, even if they stay in the same relative ranking, accrue karma faster and faster. The corollary being, if you stop commenting, you lose your position in the ranking with speed proportional to time since your last comment.

[saagarjha]

No, there are people who "should" be there who aren't. For example, 'pg with 156k karma.