Bold thread titles are supposed to be ones that I either haven't read yet or that have new posts in them, right?

When I've looked at them, they're no longer bolded. But then sometimes they are again. I'll make a new thread, it'll be bold, I'll look at it to study my handiwork, and then it'll be unbolded.

Then a minute later it's bold again, even though no one's replied.

Or other people's threads that I've already read will go from unbolded to bolded even though nothing new has happened.

Is this a bug?