It should be noted that HN's job ranking typically algorithm works by starting the job ad at #4, and using linear decay at about -1 rank every 15 minutes to have the job ad gradually fall off the front page (can't find the source unfortunately; it was said months ago)

The fact that 1) the ads appear at #1/#2 and 2) the ads are not falling down the front-page rankings leads me to believe that the issue is a bug in HN and not an intentional implementation.

EDIT: As of now, the two job ads have disappeared from the top of HN, which gives an indication that it was a bug. Software QA skills at work! :)