This confuses me too, actually. Before this feature, one's karma score was solely the sum of the scores of one's posts and comments. Adding another invisible feature means that your total karma can't be reliably reconstructed by looking at those features, even if you throw in information about the ancestors of the comments (because if you reply to a downvoted comment, that doesn't prevent it from later being upvoted). It adds an additional and uniquely nontransparent value.