My reputation dropdown indicates that I've had a change in reputation of -7 from this question, which only has one downvote. Therefore, shouldn't I have only lost 2 reputation instead of 7?
The detailed reputation breakdown (available from expanding the list item by clicking on the little arrow) shows that you both got a downvote (-2) and someone revoking his earlier upvote (-5).
You got an unupvote (-5) as well as a downvote (-2): presumably from the same person, changing their upvote to a downvote. From your reputation history (don't click if you want to preserve that number in the top of your reputation tab):