I have been told that if you reply by email to a Nabble application and change the subject line a new topic will be created (and my own tests confirm this is so).
However, when replying via the forum, the reply form offers a subject line and experiences teaches me this does NOT start a new topic.
So I suspect the answer is that some of your authors are posting via the Nabble Archive and assuming it works the same way as their mail program.
It looks like this is a case where a mail archive needs some NAML coding to make the "forum" echo the behaviour of a mail program.
Hopefully, one of the team will be along soon to make a suggestion on how to achieve that.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.