When is an impression counted in Reporting?
- As soon as a creative is requested from the publisher
- After the creative is served and viewed on the site
- After the browser begins loading the webpage
- After a user’s device indicates that the creative download has begun
Downloaded impression: An impression counted only after the initiation of ad retrieval by the publisher. The ad creative is downloaded in the user’s device and has begun to load. A separate viewability measurement will determine if the user has viewed the creative.
Read more here: https://support.google.com/admanager/answer/7308129