I don't understand why this comment is downvoted

Probably because the poster formerly known as Eugine_Nier (and by many, many other names) -- who was banned from LW but keeps coming back with new accounts -- has taken a dislike to me and is downvoting most of my comments, in some cases multiple times with sockpuppets. (My 30-day karma is currently at -159/42%, which means something like +420-579, and I think almost all of that -579 is Eugine.) His usual practice at the moment seems to be to downvote just enough to ensure that everything I post is at 0 or -1.

He has some other frequent targets, such as NancyLebovitz and OrphanWilde; if you see something from one of them with an inexplicably negative score, that may be why.