My first downvote was reported as an unupvote in my rep score. It cost 10 points. I thought that was normal until I received another downvote and discovered that it cost only 2 rep points. What is the difference between a downvote and an unupvote? I Googled it and got nothing.
An "unupvote" is the removal of an upvote, and consequently removes the rep gain associated with that vote. In my experience they are fairly rare. They are distinct from a down vote (though the effect on a question's score is essentially the same if you neglect that the voter had previously increased the questions score).
Users are allow to cancel a vote during a brief window after casting it or after an intervening edit. The user may have changed their mind becuase they read some other answers or didn't like a clarification in the comments or they may not have liked an edit that you (or someone else) made to your post after they initially voted.
When you reach 1000 rep you'll gain the ability to click on post scores and see a up/down breakdown as well as the net. Under those conditions the difference is clear.