There are two reasons why the actual space available on storage media does not equal what you think it should.
1. Some of the space is taken by the operating system and default apps.
2. The rest of the space is accounted for by the way the space is measured. Here's the explanation:
Why Your iPad or iPhone Storage Shows Less Space than Advertised
If you’ve paid attention to hard drives, USB flash drives, and other storage devices like the storage in an iPad or iPhone, you may have noticed that they always have less space than promised. The reason for this difference lies in the way manufacturers advertise their devices, versus the way computers actually use the storage devices. There’s also some overhead required for formatting and for the file system.
To a manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. Essentially, if a storage media is advertised as 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space. The hard disk manufacturer thus advertises the disk as a 500 GB hard disk. These numbers are based on tens.
However, manufacturers of RAM don’t sell it in even groups of 1000 – they use groups of 1024. When you’re buying memory, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB. The actual numbers are based on 2 because a bye of space has either a 1 or a 0 assigned to it.
To work back from the 500,000,000,000 bytes above:
500,000,000,000 / (1024*1024*1024) = 465.66 GB