well it also happens, if you post/reply in a thread which is going to be deleted, such as duplicate topics, non-sense topics, so better not to reply in such topics as when that thread will be deleted, your posts will also get deleted.
1. you post / reply on a thread that has been posted before (double thread). All double threads will be deleted from database, and of course all members who make a reply on that thread will lose their post count as well.
2. you make a reply on a thread which was posted on a wrong section, some 'wrong section" threads will be deleted and you might lose your post count.
3. you make a reply on a thread which is now moved to non-counting section.
4. your post is considered as spam or unnecessary post.