Just Curious - Why Are Only Some Stories Nofollowed?

[cvos]

I love reading hackernews, but just noticed a Curious quirk. The story links appear to have random nofollow tags assigned to them. In this screenshot, 4 out of 30 homepage links are nofollowed. http://www.img7.com/images/hakckernew.jpg

I'm just wondering if this random or editorially controlled.

[mooism2]

They're all links that have collected fewer than 10 votes. (I don't know what the exact threshold is.)