Per this MSE, posts are enqueued into Late Answers if they were posted more than 30 days after the question and their owners have less than 50 reputation. This was not always the case; originally the rep threshold was 10 points. Here's what I think happened:
- The new user posts the answer several months after the question was created
- It swiftly receives an upvote (see timeline), putting the user at 11 rep, above the old 10 mark
- The answer accumulates four more votes, eventually putting the user at 51 rep
- The Late Answers owner reputation cutoff is changed to 50 points
- A user who voted for that answer gets deleted, removing 10 points from the post owner
- The system notices that the post qualifies for Late Answers but hasn't yet been reviewed there
The vote count of 5 in your screenshots seems to have been cached — I clicked to see the vote split and now it says 4. This and the user's rep graph made me suspect voter user deletion, since the post has never been edited.