That doesn’t seem possible by one person; the max strong-upvote amount at the moment is 9. There’s no downvote-upvote combination that leads to a difference of 13 in the change of 1 vote (though it would be possible for someone to strongly upvote and then change their vote to a strong downvote for a karma drop of 14,16, 18). A more likely explanation is that multiple people downvoted while (multiple people-1) retracted their upvote within the time you were refreshing, or some kind of bug on the forum.
That doesn’t seem possible by one person; the max strong-upvote amount at the moment is 9. There’s no downvote-upvote combination that leads to a difference of 13 in the change of 1 vote (though it would be possible for someone to strongly upvote and then change their vote to a strong downvote for a karma drop of 14,16, 18). A more likely explanation is that multiple people downvoted while (multiple people-1) retracted their upvote within the time you were refreshing, or some kind of bug on the forum.