I've noticed something odd over the last few weeks, the image ads provided by adsense seem to be matching the theme colours of the site.
Thus if you have a site with pink as a theme, you are more likely to get pink ads. If your site is blue then ads are also more likely to be blue.
Most of the time the ads are reasonably on topic, but there seems to be more topic variation than there used to be. Sometimes I get something that is right off the wall - ad is a perfect colour match for one of the main images on the page but the topic is way off what I normally get.