A new change was added to AdSense tonight, regarding the number of ad appearing in an ad unit. If the AdSense algo believes showing fewer ads is more advantageous to you, it will drop an ad and expand the remaining ads to cover the entire ad unit. So an under-performing or lower paying ad could be removed from an ad unit under this new change.
To increase monetization on your site and improve the relevance of ads, AdSense now varies the number of text ads that appear in a given ad unit. In cases where we determine that increasing the size of the most relevant ads will improve performance, we'll drop the lowest-performing ad or ads and expand the remaining ones to fill the entire unit. Showing fewer ads works to your advantage, allowing the better-performing ads to draw more user attention and click-throughs. Google AdSense technology will automatically determine the optimal number of ads to display on any page and will only show fewer ads when doing so will make you more money!https://www.google.com/support/adsense/bin/answer.py?answer=22506
The biggest impact this change will have is to those publishers utilizing images above or beside ad units to draw attention to and showcase the ads. Because the images are generally lined up to appear with each individual ads in an ad unit, this new change will mean that the images no longer line up with the ads, if the number of ads in an ad unit changes.