Mobile Ad Selection
Mobile ads don’t look the same as desktop ones. Ads you want to display on a mobile may not be what you want the desktop users to see. Ads perform differently whether they are on mobile, desktop or tablet… you get it.
What you can do to fix it:
Google actually allows you to select which ads you would prefer to show on mobile. If you don’t select anything, then all ads will be treated the same so it may be worth thinking about differentiating mobile and desktop ads.