Sometimes I get them..
and the ads there have a cookie in them. I end up getting ads from amazon too. they are remnant ads, which show when you've seen all the premium ads.
basically most ad campaigns that we get decent money for are capped at about 5 ad showings per day, per user. This is basically to ensure ads don't get shown to one person 1000 times, and another person once, and then the campaign ends (although most people buy a minimum of about 50k showings of their ad, and about. It's probably also to ensure cheeky nerds don't just set-up a thing that does loads of pageviews, to try to earn more ad revenue.