I don't consume HN via RSS, so I am just speculating, but perhaps the RSS feed has a lot of turnover. HN on the web is nice because links stay on the front page for a while, but a chronological delivery such as RSS might be messier.
The "messy" nature of RSS feeds is an advantage when dealing with sites like HN since you're not affected by any type of ranking mechanism, i.e. you get to see those posts which for some reason or another are quickly moved off the front page.