I was looking through my stats for Ezine Articles and I noticed something interesting. The articles that I'd submitted related to internet marketing generally hadn't accrued page rank themselves. But some of those that went into other categories like "pure opinion" and "movies and TV" had. And this wasn't just those I'd submitted a long time ago; they'd only been up there a couple of weeks.
I suspect that this is mainly due to the numbers. That is, the internet marketing and seo niche is pretty well saturated with articles, since every money-making webmaster and his dog is busily building backlinks with them. But those other niches are simply less full, and so the average page rank for each article is higher.
Whatever the reason, it's something to remember. You should tend to get better SEO results from your author bio-box links if you submit to the less saturated niches at Ezine Articles - and probably other article directories as well.