Google AdSense announced a new feature to “Fill empty in-page ads” by showing “contextually-relevant suggestions related to your page and site inside unfilled ad spaces.” In short, if an ad is not filled by Google on your page, then Google will look for other ways to monetize that ad spot.

I should note at the top, that Google will turn this on by default for all AdSense publishers within 30 days. So if you want, you can opt out of it.