Because the ads served are not handled by SMC's server; rather, they are provide by a third party and it uses any available data to personalize many ads served to each visitor. So since you were browsing a particular site recently, the third party ad server recognized that and provided you with a, um...reminder...that you expressed some level of interest in the same. This is the way it is on most sites...they don't sell individual ads so much. Sometimes banners from specific advertiser partners of the site will appear in the rotation and that is easily provisioned.