I was playing around with the mod tools (testing out some bugs and stuff) when I noticed that I'd caused this:
This occurred because some combination of mod tools added an undelete vote, marked the post as undeleted, and then didn't clear the undelete vote, causing it to be re-applied when the answer was re-deleted.
As useful as this is, moderators aren't supposed to be able to cast ordinary votes, and there are no plans to make this an option. Sadly, this is a bug and not a feature.
(CMs, see here for more information.)