If you've made a lot of posts in a thread and it gets trashed then your post count will go down. Don't post in unsubstantial threads that are liable to be deleted if you're worried about this.
I don't have any problem with the cleaning of similar or shit threads, moderators are doing there job well for the betterment of forum but it will be more better if they compile a list of trashed threads so that everyone should aware of those threads and try to avoid those kind of threads in future.