From recent Google’s experiment, we see positive revenue improvements for the top anchor ads for publishers.
Anchor ads stick to the edge of the user's screen and are easily dismissible. These ads mainly serve on mobile but can appear on desktop too.
Anchor ads appear at the top of the user's screen unless we determine that this would negatively affect the user experience, in which case we show them at the bottom. When shown at the top, anchor ads only appear after the user has scrolled down far enough, so they never cover your site's branding or navigation.
If you'd prefer to only show anchor ads at the bottom, you can disable this functionality by changing your ad code.