I think that I've seen before at least one user being banned and all their posts being obliterated. But I don't remember details.
Moderators use the "nuke" feature if they want all the posts to be deleted and the user banned, but I don't think this is what caused the wrong post count.
According to
modlog,
this user is the last user who has been nuked. As you see in his/her profile, the post count is correct.