Why is there a difference between "Total ads served" and "impressions"

Why is there a difference between  "Total ads served" and "impressions"?  
 
"Total ads served" is the number of your ads we give to publishers
Impression is when we see the impression beacon fire impression is what we bill from.
 
The difference can be due to a number and combination of reasons:-
 
1. The publisher is running an internal auction and sometimes does not use the ad we serve
2. The user moves away from the page where the ad destination so it is not used
3. When the impression beacon does fire we decide that it is invalid activity
4. There is a network issue and we do not see the impression beacon fire so we don't count it