Regarding general hard drive and iPhone and what have you storage devices showing less than their manufacturer-stated capacity, the reason for the difference in the numbers is very, very simple. The reason has to do with marketing, and with simplicity in storage device manufacturers labeling their products. Here it is:
All computer devices are manufactured for, & operate using, the binary numbering system; i.e., the Base 2 numbering system; i.e., 1's and 0's. However, for simplicity's sake more than for any other reason I suppose, manufacturers *report and advertise* their storage devices' capacities in the "normal" everyday-use decimal numbering system; i.e., the Base 10 numbering system.
So when a memory storage manufacturer builds an iPhone's memory for 32GB, or a desktop or laptop hard drive for, say, 1TB, then the resulting Base 10/Base 2 storage capacity calculates out to, for the 32GB iPhone, as 32,000,000GB total stated Base 10 amount ÷ 1,048,576GB of similarly close Base 2 amount per Gigabyte = 30.517578125GB. For the 1TB hard drive, that'd be 1,000,000,000GB (Base 10) ÷ 1,048,576 (Base 2) = 953.67431640625GB real, actual storage space amount.
Partitioning and formatting reduces that "real, actual storage space amount" just a bit more.
So there it is..... Marketing, a difference in the Base 10 "human" numbering system and in the "computer" numbering system, and simplicity in labeling.
Kevin Kendall
Ol' tried & true & still good 7,1 2.4GHz white Macbook / 1TB HDD / 16GB RAM / Sierra beta4 with Siri (yayyy!!) / Windows 7 & 10 via VMWare 7