It is because storage drive capacity is calculated and reported slightly differently than other capacities in computing. If you look at the specifications on the back of an iPhone box you will see a note that says something along the lines of ”1 GB = 1 billion bytes. Actual usable capacity may vary.” In other words, the drive capacity is reported on the assumption that 1GB is 1,000,000,000 bytes. So 128 is in other words actually 128,000,000,000 bytes.
If you are familiar with computers you know that they calculate things slightly differently. Computers count 1,024 bytes per Kilobyte, 1,024 KB per Megabyte, and 1,024 MB per Gigabyte. This means that when you install a 64,000,000,000 bytes storage drive into a computer, that computer converts the amount of bytes into gigabytes by dividing by 1024 all the way up through the scale, not by dividing by 1,000.
Doing the math, this is what we end up with:
64,000,000,000 Bytes / 1,024 = 62,500,000 actual Kilobytes
62,500,000 KB / 1,024 = 61,035 actual Megabytes
61,035 MB / 1,024 = 59.6 actual Gigabytes
And that is why a 64GB will be correctly reported by the iPhone as 59.6GB. The larger the numbers are, the larger the discrepancies will be. On an 8GB USB drive the difference between the advertised capacity and the actual is about half a gigabyte, while in our example above the difference is a very noticeable 4.3GB. It is important to understand that these 4.3 GB aren’t lost. The drive is 64,000,000,000 bytes in capacity, and after 64,000,000,000 bytes have been converted into Gigabytes, the total capacity comes to 59.6 GB.
now add the OS and you are down to your 55GB.
