Looking at it again, I don't think the law idea I put forth makes sense. The pricing plan sounds more likely, especially given the ringing behaviour at multiples of 30/31 days.

I guess a large majority of those postings go unfilled.

Do you do any work to try to find if a position has been re-posted and recombine it with its previous posting for the purpose of this calculation?

I don't know if one can draw the October conclusion. Would HR leave the posting up to expire on their own if they know they won't hire anyone?

I say that since they seem to remove them (presumably because they got enough applicants or actually hired someone) before the 30/31 day mark.