Ads get classified via their ad network's sales process and get nominated into a number of categories to offer brand safety to publishers. Ezoic offers ad blocking via our UI. Our system integrates with a number of ad exchanges and we ensure best practice elimination of key categories to all our publishers, so that you can control the types of ads that are being shown on your sites.
Ad classification is not an exact science (see below for a more in depth view on this aspect of the online ecosystem), but works well for the vast majority of publishers and advertisers.
Putting it simply - the online landscape behind the ads we see on sites is hugely complicated mesh of ad networks, ad exchanges, brokers, data purchasers, retargetting specialists and ad agencies. Billions of ads are served hourly. Ultimately, online advertising relies upon tracking performance. Users need to be seeing and clicking on ads and then - ultimately - completing an action that keeps the advertisers happy with their ROI. As a publisher, your users are constantly being assessed for their traffic quality (propensity to buy/do something). The advertisers then continue to pay what they think inventory is worth and bids increase or decrease over time as a market rate is found between supply and demand side platforms. Ads are either bought speculatively in the form of cpc or cpm ads - backing it out to an effective cost per acquisition, or paying a cpa rate post click (aka affiliate). Even so called 'brand ads' are often purchase with cost per engagement or other performance metrics/indicators attached..
For this ecosystem to continue to grow, brands need safe sites to advertise upon and publishers need safe brands to show their ads to; it's a symbiotic relationship.
Just as sites get categorized for their content type, ad networks like Adsense have feeds of advertisers from their sales arm (AdWords) and rely on categorization of the ads to help target users who might be interested and keep the ads on the appropriate pages. This means each ad gets reviewed and categorized. Whilst Google do an amazing job of this, other ad networks who bid for inventory in ad exchanges may not be so fastidious in their classification of their advertisers. With the volume of ads being sold hourly - there can be the odd one or two ads that get through the screening process. On the whole, an ad block will work 99.99% of the time, but not every ad is perfectly categorized; it's just the nature of the online ad industry. The reality is that we cannot deal in absolutes as much as we would like, so whilst we do everything we can to ensure an ad block works as well as it can; it's not something we can give a cast iron guarantee for (no one can).