As far as I understand it you mostly don't see it because once a thread gets to the top it is receiving a net positive stream of incoming vote and since it is already at the top it will receive more votes that threads further down the page.
Sometimes threads are detached by admins and end up at the bottom of the page. In those cases they are usually (possibly always) clearly marked as such.
Perhaps that's the case, but isa's comment was the first comment for 4 hours after it was posted. Round about two hours ago, it suddenly dropped down to the fifth slot, not the second or third.
I don't know what HN's policy is on resequencing; I thought I would ask to see if I'm not understanding something correctly.
As far as I understand it you mostly don't see it because once a thread gets to the top it is receiving a net positive stream of incoming vote and since it is already at the top it will receive more votes that threads further down the page.
Sometimes threads are detached by admins and end up at the bottom of the page. In those cases they are usually (possibly always) clearly marked as such.