iCap = 64GB *10^9 B / 2^30 B as GB
iCap = 64 GB in 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
The iOS operating system divides the total disk capacity into two parts:
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
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
This means that more storage is apparently lost in a unit conversion (4.4GB)
than what is lost to iOS, built-in apps and some overhead. (2.5 GiB + 1.4 GiB in 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.