This email didn't do a lot to set my mind at ease:
We're writing to let you know about an upcoming series of refinements to the contextual ad targeting system that should increase the relevance of ads that appear on AdSense sites. As you may know, contextually-targeted ads are matched to your site's content so that they're useful and relevant to users.
While contextually-targeted ads make up one of several components of your earnings, we've identified that this change may have a noticeable negative impact to this portion of your revenue. Ads will become more relevant in the long run, but it's possible that fewer ads will be available to show on your site.
We believe these changes will help advertisers improve the performance of their contextually-targeted ads and encourage them to bid more confidently on AdSense sites. While there's no guarantee of how this change will affect your revenue in the long term, we encourage you to continue developing useful and valuable content for your users.
Thanks for your understanding.
The Google AdSense Team
My brain interpreted this email as:
Blah blah this change may have a noticeable negative impact blah blah.
Google's PR department sells the story about how Google tests everything scientifically to make the entire Google experience perfect. I don't think they know about emails like this one.