When I hover over the upvote or downvote arrows on a post where I have voted, I see the following tooltip "this answer is useful (click again to undo)".

[![highlighted vote up tooltip][1]][1]

I never really noticed the phrase "click again to undo" as I always knew that I could do that. I also knew about vote locking after, I think, 15 minutes.

So, I thought that this message meant that my vote could be changed because of an edit. However, there had not been any edits on that post, so I clicked the vote button to check the message and saw: "You last voted on this answer yesterday. Your vote is now locked in unless this answer is edited."

[![your vote is locked in message][2]][2]

So my question is: why am I being told that I can undo my vote and then told, in fact, that I cannot undo my vote?

Shouldn't the phrase "click again to undo" only appear when I can, in fact, undo my vote?

  [1]: https://i.sstatic.net/B3o7l.png
  [2]: https://i.sstatic.net/tmAPB.png