I don't believe the threads actually get moved. "Show HN" is simply a filtered view, so the same thread could be on top of Show HN and on the first page of HN.

I don't believe the threads actually get moved. "Show HN" is simply a filtered view, so the same thread could be on top of Show HN and on the first page of HN. However, it would be possible to hide the "Show HN" when the thread is shown in that view, although I don't know if it's all that useful.