Karma points count as "last 30 days karma" if they are votes on a post you made within the past month. If someone upvotes/downvotes/removes a previously made upvote/removes a previously made downvote from an older comment, you get/lose karma, but not 30 day karma. I assume that is what happened here.
Yesterday I had 25 karma, having earned 15 (100% positive) in the last 30 days. Today I have 24 karma, still having 15 (100% positive) in the last 30 days. What is happening here?