The grey circle, the blue circle
Clicking the blue 'New posts' button links to the first unread post, rather than the first new post.
Two circles to display unique information, both link to the same place.
That's because the circles are just indicators and not built to be links to unread posts. They're just links in general for convenience.
I'm sure that's what the argument is for them.
Filed under: Just following the general theme of Discourse Usability, I presume.
Just like NEW is written in blue, in the same manner as old 'cold' topics are marked in blue.
Reply is blue.
Links are blue.
Tags are blue.
Scrollbar is green.
The header is blue.
Unless you're on mobile replying to a user, reply is black.
Old topics are blue too