The answer ultimately boils down to the AdSense ad auction, and is a way to maximize your revenue. As a refresher, in the ad auction, advertisers bid to show their ads on your site: The number and price of ads in the auction changes from impression to impression, based on how much advertisers are willing to spend and how they’ve set up their ad campaigns. This is why we call the ad auction ‘dynamic’. In addition to their bids, advertisers are also ranked based on their Quality Score to ensure a good user experience.
To help you understand how our system selects between different ad types, today we’ll first focus on how text and image ads compete within the same auction for your ad space. Let’s say you have a 728×90 leaderboard and have enabled both text and image ads to appear. For every impression, our system predicts which ad type will be more valuable. When selecting the “winning” ad(s) for your page, our system takes the advertisers’ bids and Quality Scores into consideration and also considers historical data for specific ads, in order to predict overall performance.
So, even though a leaderboard could show up to four text ads, it may sometimes be the case that the one image ad will outperform all of the competing text ads. In other words, if you see one image ad appearing in your leaderboard, it’s because it is more valuable than the eligible text ads combined. By maximizing competition for your ad space, you’re also maximizing your revenue potential — this is why we always recommend enabling both text and image ads for your ad units, which maximizes competition for your ad space.