Couldn't that just be due to a higher number of total votes (both up an down) for the OP? I would assume fewer people read each comment, and downvoters may have decided to only weigh in on the OP. A hypothetical controversial post could have a karma of 8, with 10 downvotes negating 10 upvotes, and a supportive comment could have 9 upvotes due to half of the upvotes of the first post giving it their vote. The comment has higher karma, but lower volatility, so to speak.