I don't know if it happens all the time, but sometimes I've noticed that if you edit a comment, it'll add a +1 to the comment counter. Which in the case of that thread, it says four comments, but there's only three and [email protected] has edited their comment to make it four.
I think it's a bug when people from outside the instance where the thread is hosted edit their comments.
A recent example I noticed is that if I go to this thread it shows 8 comments (viewing from Lemmy.ca) but if you view the same thread from Lemmy.world it shows 9 because I edited my comment.