the private message is indeed between poster and receiver. But if this post is send to many other receivers in the same time. Every time someones adds a reply it will be seen by all others who received this p.m post too. and it will show for you , you have a p.m. again.
I just went back and looked again and it has 81 replies on it and none of them mine.
I thought the only ones that could be on my private message site was me and whoever I sent a message to or whoever sent me a private message???????
MJDG