ELI5:How come a device with a HDD/Storage gets advertised without accounting for the System use?

For example a 16Gb Iphone, only has 12 or 13 Gb of storage after the operating software has been installed. Also, who not just make it so you actually get the advertised storage(making the actual storage 19 or 20gb)

Submitted December 01, 2014 at 06:28AM by SlusheeBoy http://www.reddit.com/r/gadgets/comments/2nw6g2/eli5how_come_a_device_with_a_hddstorage_gets/