"Application installs" stat reflects all the unique devices registered in our system for a certain app during the selected period. However, not all the users subscribe for push notifications. Therefore there is another "Device register" stat, which reflects the actual number of such users.
"Device Register" — no. of devices, not necessarily unique ones (the hwid might have already existed in the database) subscribed for notifications during the same period.
"Device unregister" stat, in general, is the number of users unsubscribed from notifications. But the reason may vary – the application was deleted, user unsubscribed from pushes, or the push token was invalidated.
Please note that 'Device Unregister' stat reflects not only the unregisterDevice requests, i.e. action of users, who unsubscribed from notifications themselves. Devices can unregister for a number of reasons, such as invalidation of tokens for example.