I'm certainly glad it was posted this particular time. I've never seen it before and now I learned something today. :)

I'd also never seen it and find it very useful.

Confirms what I expected - not new in the world, but new to many/most people.

Does anyone know the probabilities of HN moving between states “animated Markov chains article at the top” and “animated Markov chains article not at the top”?

[[0.0,1.0],[1.0,0.0]]

Had to be said :)

There is definitely memory, but HN also has new members. If repeat topics move up to a top location, this probably reflects the number of people who hadn't yet seen the topic. "Not new, but new to you."

Even without new members, not everyone is on all the time, so some people miss stuff like this. Or, even if you've seen it again, you vote it up, because you like it and want others to see it.