The average impressions for a Story is the Impressions divided by the Reach. That is to say, the average number of views divided by viewers for this Story.
The Impressions are the number of times your Story is viewed. One person can see your Story multiple times, and be counted each time.
For example: 200 people see your Story once, and 100 people see your Story twice, therefore your post had 400 impressions.
Wondering how your Reach calculated too? Here is the answer :)