[pvg]

http://www.raterush.com/pages/digg-reddit might explain this somewhat.

A huge chunk of reddit links are back to reddit, to imgur and to things like major newspapers and youtube. If only a few of these are not actually tracked by Woopra, the results would get skewed significantly.

[ArcticCelt]

Then for the few legitimate remaining articles, I'll postulate that reddit users go to comment after only reading the headline and never follow the link of the article. Maybe HN readers are more diligent and do read/visit the articles more often.

[vaksel]

that's just the front page though. Reddit has a ton of subreddits...which people subscribe to. When they do, the stories from that subreddit show up on your main page.