My guess is that the target audience is diffrent therefore the motivation to click on ad changes.. Most people who are on digg are webmasters who want people to come to thier website so they won't click the ads. On plentyoffish- the people going there are looking for dates and so they are more likely to click an ad regarding dating from that site..
Yeah thats an easy one. Digg = mostly web savvy people who tend to know what adsense is. POF = most clueless to adsense
That and plentyoffish is so poorly designed and ugly that people will click on anything just to leave.
Plentyoffish is a dating website, isn't it? I'm sure a lot of people are looking through profiles and all that.
a great thing about dating websites is their traffic soars on the weekends... something most of us are not used to!