Post circles showing up on threads set to "normal"

In an attempt to clearly and rapidly identify topics I’ve posted in when checking the “latest” list, I have my preferences set to leave topics set to “normal” no matter how long I read them, so long as I haven’t posted in them. And my preferences are still set that way, and topics I’ve read but not posted in are still set that way, and what shows up on the “unread” page is only the topics I posted in – but when I look at the “latest” page, not all along but recently I’ve been seeing those blue circles with a post count in them on topics that I’ve definitely never posted in. As I only sometimes remember for sure whether I’ve posted or not, this leads to my thinking that I’ve posted in topics I haven’t posted in.

Why is this happening, and can it be stopped?

ETA: Firefox on an old Mac desktop.

ETA 2: and why is this thread showing up as greyed out on the “latest” list?

Happens sometimes. Refreshing the page usually gets rid of the blue bubbles.

I think that just means that you’ve read the whole thread and nothing has been added since you last visited it.