Things like computers and phones have programs and software already installed which do eat into the available GB. Losing 4GB seems a bit high to me, but you could go through your applications and see if you can uninstall anything.
Up to 50% available memory can be reserved for system files and recovery partitions. I believe the iPhones may take up about 3-4GB of user storage. IOW, a 16GB iPhone may have only 12GB usable storage due to immovable and inaccessible files and OS system requirements.