When you view a conversation with unread messages, or even if the person in the opened conversation is messaging you, the unread counter will not be cleared, and will increase with each new message.
This is a duplicate of #73, some there suggested that messages should be marked as read as soon as the conversation is clicked on. I think it is worth following it there
When you view a conversation with unread messages, or even if the person in the opened conversation is messaging you, the unread counter will not be cleared, and will increase with each new message.