Why are my impressions less than my subscribers

There are many reasons why the delivery rate (impressions / subscribers) is not 100%. It gets affected by a range of factors: 

1. Browser settings - push notifications are sent through APIs from the browser / an installed PWA (which also uses browser APIs). If a user uninstalls the browser they used for subscription, then they would not get this impression. Similarly while many browsers allow sending when the browser is closed, some do not support this functionality
2. Device settings - Phones on which low power / battery saver mode is activated or where background activity is disabled (even temporarily) will not get push notifications. 
3. Site data - Customers who reset cache / cookies / clear site data will not get push notifications until they revisit the site. The browser saves data from the subscriber's device, and this enables push notification delivery. If the history, cache, or site data is cleared, the subscriber's preference to subscribe to your notifications is forgotten. If they visit the site again, we can resync this information and they will resume getting push notifications.  

These are the most common reasons. Based on research and our own experience, we see between 20-50% delivery rates (also depends on the mix of subscribers on desktops / mobiles / tablets). The best way to increase effectiveness is to have a large subscriber base and good content. The larger the incoming flow and subscribers, the more impressions, etc.