@iPhone's Capacity
"Storage device manufacturers measure capacity using the decimal system (base 10), so 1 gigabyte (GB) is calculated as exactly 1,000,000,000 bytes. The capacity of the storage media in your Mac, iPad, iPod, iPhone and other Apple hardware is measured using this decimal system." [1]
Apple, and every other manufacturer, uses 1 GB=1000^3 bytes (decimal system) when talking about the hardware, but they use 1 GB=1024^3 bytes in software (binary system);
@Confusing Units
In the case of an iPhone '6s' with '64 GB' on the box, this means it would advertise '59.60 GB' in software. Mathematically, if we state that 'iCap' is the total capacity of the iPhone in software (binary), then;
iCap = 64GB *10^9 B / 2^30 B as GB
59.6 GB
Or represented, more correctly, in its internationally agreed upon binary unit, the Gibi-byte (GiB) [2] ;
iCap = 64 GB in GiB
59.6 GiB
This amounts to an apparent (but not real) loss of 4.4GB (64 - 59.6 GB), which in reality is no loss at all, just a miscommuncation:
Because, 64 GB - 59.6 GiB
0 GiB
@iOS overhead
The iOS operating system divides the total disk capacity into two parts:
1. the system data part (the core operating system);
2. the user data part (apps, documents, music, …);
It reserves around two and a half 'software Gigabytes' or 'Gibibytes' - (2.5 x1024^3B as GiB) for the system (part 1). If we subtract that from our remaining iPhone capacity iCap, we get:
iCap = 59.6 GiB - 2.5 GiB
57.1 GiB
Finally, there is some overhead for the filesystem and there are pre-installed apps that take up some of the free space on the data part (part 2) I didn't verify this, but the remaining 1.4 GiB doesn't sound unfair or inaccurate at all. Bringing us to the 55.7 GiB that Apple lists as the total capacity in its "About this Phone" dialog:
iCap = 57.1 GiB - 1.4 GiB
55.7 GiB
This means that more storage is apparently lost in a unit conversion (4.4GB)
4.4 GB
than what is lost to iOS, built-in apps and some overhead. (2.5 GiB + 1.4 GiB in GB).
4.19 GB
I'd conclude, there is no trickery going on, but Apple, as an industry leader, has the opportunity to be more clear on this. At least they should use the GiB unit demarcation in software, because that's what they are using.
// [1]: https://support.apple.com/en-us/HT201402
// [2]: https://en.wikipedia.org/wiki/Gibibyte