Ads are keyword, key phrases, title, and importantly hub success orientated. An advertiser contracts with Google or Adsense. An advertiser is willing to pay for results within the market they are pursing. They set parameters with Google and Adsense. based on their criteria and the formulations of Google and Adsense for ad placement. It is how wide the artists brush is. Broad or narrow that is of concern.
Example: I wrote financial articles once. The ads were high value jewelry - diamond rings and etc, high value car ads (Lexus and such), resort vacations, financial institutions, and flying. As my article lost viewers the ads changed away from that narrow demographic market becoming more broad. In other words my article did not meet the criteria for those advertisers with a narrow market and the dollars (however that is calculated) that they wished to spend because of article popularity. It costs the advertiser to advertise. No results means no ad.
Example: I wrote articles about writing with emotions in fiction. The ads placed on those were education based ads, self-help organizations / businesses, and freelance writing. One emotion I wrote of was Love. The article was about writing with that emotion. Some of the ads were for cards and literature for Valentines day, poetry orientated ads, and internet flower businesses.