Some sites have more campaigns/ads in rotation from fastclick.
This could be due to the content category of your site, that you have deselected many campaigns, or simply that Fastclick has not matched your site with many advertisers yet.
Most likely what you are seing on these other sites are chained defaults from lower tier networks. Fastclick allows you to set up default campaigns which will show when they don't have paying ads to display. You can use this feature to run ads from other, lower-paying networks when there is no paying ad from FC. This is generally seen as a much better use of your inventory than showing Smokie the Bear and other AD-Council provided units.