Why lower eCPM?

Google Adsense has updated their blog once again with more information about the new ability to compare the contextual and site targeted CPM ads.

It seems that many people are a bit concerned about site targeted CPM ads showing a lower eCPM as normal contextual ads. The answer? It's perfectly normal. The CPM and contextual ads bid for the same ad spot on your site. Adsense ensures that the highest paying ad is displayed depending on specific ad spots and pages... so Google isn't doing you in.

The reason why the eCPM might appear lower is because the site targeted CPM ads are most likely being shown on the lower performing pages of your site. That means that the CPM ad unit doesn't have to bid as high a price as a premium spot would cost.

Best of all seeing as CPM ads are paid by impression, you are getting revenue from pages that might not have earned you anything. Sounds like a pretty good deal to me!