That's it cj.
I find it a bit confusing too, especially when I'm figuring out which posts to read and which are old.
Coupled with that is the fact sometimes my screen shows all threads as read even when they're mostly new and unread, and some threads as new when I've already looked at them.
The time signature is helpful for keeping tabs on whether there's something new added to a discussion I'm interested in, beyond the coloured globe changing colours, which doesn't appear to be 100% reliable