@sam, I do understand what you’re saying and how you’re explaining it (which makes sense and is very clear), BUT this doesn’t seem to match what I see happening here on Discourse 
Very last example: Just now, I have been shown 29 unread posts in this topic: Discourse BBCode . I’m pretty sure there wasn’t ANY unread messages last time I was here (not that long ago), and now I see OP + only 2 posts from 2 hours ago (total messages in the topic = 3). I don’t think there were 27 new posts created and deleted since my last visit, or was there ? (that would be a simple explanation !). Could you please have a look ? (If not, why did a “29 unread” blue bubble appear ?)
Maybe this is linked to these kind of topics: “Replies on this topic are automatically deleted after a month”. Or maybe it is linked to when the user started to follow the topic ?
I’m a little lost here 