I saw another thread about this, but could find it. I've never been on the other side of the equation (i.e. AdWords), but it makes sense that as an advertiser I would pay less if I have to share the page with someone else. And I would pay more if I kept it all to myself. Maybe someone using AdWords can shed some light on the subject.
I did a test before. I had more impressions but the value went way down. So i took 2 of them back off. The first ad unit suppose to have the highest value clicks.
I think the highest bidders get listed first; thus it would make sense that the fewer ads the higher you get paid per ad. That said, I'm not sure if you'd make more money overall or not.