Generally your push notification "sent" count may be lower than the count of your target "segment" because of two reasons:
Some of your notifications "bounced". Essentially means that we were unable to deliver that notification to the subscriber - usually because they have unsubscribed. Our system will automatically attempt to deliver the notification several times if it fails and if it's still unsuccessful, we will automatically remove that subscriber from your lists.
You added your GCM/FCM keys incorrectly. If this is the case, contact us and we will make sure it's all set up properly.