I am an Adsense Publisher for several years. Usually I carefully try to use the new features.

Most probably in last year, adsense introduced adsense optimization report. Each month, it shows an report of previous month. If its May, 2008 they will show April 2008 Optimization Report. After clicking on the link, I got two maximum reports. One Placing more than one ad unit on a page often generates more revenue.
Many of your sites only contain one ad unit.

and second You may be filtering ads that monetize well on your site.