Back to "Go To First Unread Post"

PlanMan asked about this in this thread, but did not seem to get an answer. I’m curious, too, so I am restating the question.

** Why does the “Go To First Unread Post” link appear at the top of some threads and not others? Is it put there by the moderator of a particular board, or at the whim of any moderator passing by? And why is it put at the top of a thread with one response but not the ones with many responses.

Particularly odd is when the link is there but the thread has only one response: I can just scroll down and BIP!, there I am at the bottom of the page (and can still see most of the top of the page to boot).
Other times there is no link but 42gadozen responses: I have to wade through “read that, read that, read that, read that, read that, read that, jeez what an idjit, read that, read that, read that, hahahaha, read that, read that, still an idjit, etc. etc. whoops missed it, [scroll up, scroll up, scroll up] Ah Finally!”.

What is the criteria for this link?

[Or am I be terminated for seeking information from the MM-C (Moderator Manual - Confidential)?]