Looks like the per post rating is a whole new feature in addition to the general score, which is just like people's overall opinion of you irrespective of individual posts. Now people can give positive or negative feedback on individual posts, and that rating also increments the total Karma score. The downside is that there's no limit to how many times an individual user can up or downvote a given post, so as with general Karma it's pretty easy to inflate the rankings.