What you're probably seeing is Amazon's new "Omakase" ads. They've been beta testing them for a little while. Apparently they take into account the user's Amazon search history (have you searched for widgets on Amazon or affilates?), the site's referral sales history (have ads on the site resulted in widget sales before?) and the site's content (is the site about widgets?). I've been using them on my blog -- not sure if they are more or less effective than regular ads, but I do like that their appearance is more customizable and can be made less in-your-face and more Google Adsense like.