My understanding is that the actual HN implementation is heavily customized and not public.

Maybe what's hidden is the code for determining what counts as a vote. e.g., multiple votes from the same IP may be discounted or simply counted as a single vote. Or a bunch of votes coming from fresh accounts created at or after submission time.

That algorithm has been in the source for a while. What is unknown is the current constants and if he changed it at all since first publishing it.