Google AdSense draws ads from the Google Ads programme to display on your pages. Even though the number of advertisers in the Google Ads programme is growing continually, Google might currently only have a limited number of ads that are targeted toward the content of any specific page.
This might result in less than the full number of ads, or a reduced rotation of the ads on your site.
Additionally, the ad auction allows advertisers to compete for and win multiple ad units on the same page. So your visitors might see identical ads from the same advertiser on your pages.
For example, let’s say that you’ve placed two ad units on your page. Depending on the outcome of the auction, the same ads could appear in both of your ad units.
This ensures that your site has access to as much ad inventory as possible, increasing value for you, your users and the advertisers.