I think "Ask HN" articles get special treatment in the ranking system.
http://news.ycombinator.com/ask
Here's what it takes to define such an operator, in case anyone's curious:
(newsop ask ()
(listpage user
(msec)
(retrieve maxend*
[and (>= (realscore _) front-threshold*)
(cansee user _)
(blank _!url)]
ranked-stories*)
"ask"
"Post Without Urls"
"ask"))