I would theorise that the only reason you expected the back button to return you to HN was that your expectations have been poisoned by having to deal with the broken behaviour of sites like this made in Flash rather than HTML5.
Well, the site is basically a few pages of content split into an endless number of excruciatingly tiny chunks. Each 'page' is really a paragraph (and doesn't actually entail a page load) - nrmally I'd expect 'back' to take me back an entire page, rather than a single paragraph.
If you're going to use ajax to slice pages into tiny sections, it's far more usable if you then make the back button act as if you hadn't done such a stupid thing in the first place.
http://news.ycombinator.com/item?id=1883654
Online discussion confuses me.