How do you define total impressions on Activity reports?
The total amount of impressions refers to the number of times users have seen a post, tweet, or even an ad of yours. A user may see a post multiple times and each time counts as another impression. However, once an impression has been recorded, it it does not add to your reach. This is because reach counts the number of users who have seen your post, while impressions counts how many times it has been seen.