What is the difference between Site Impression and URL Impression in Google Data Studio?

For impressions, if a property shows twice on a search results page when accruing property, it is regarded as a single impression; if grouping by page or search appearance, each page is counted separately. For position, when collecting property, the topmost position of your property in search results is reported, when grouped by page or search appearance.

Because of the different accounting procedures, the click-through rate and regular position are higher when amassing by the property if multiple pages from the same site arrive in the search results. For example, if a search was for “fun books for children,” it could come up with many different results on the same site.