In the developer dashboard, under stats, we can find two types of graphs: installations and impressions. What does these mean?
The impressions are the number of times the extension has appeared in search (both the Web store search and Google). The user does not have to visit the page to register an impression and if the user goes directly to the page via a link in an email etc. it will not count as an impression. This is why my impressions are less than my installations (people send links on Facebook and twitter a lot more than they search for it).