Have you bought a new flash drive or hard drive and once you plugged it in you saw that the memory available was less than advertised? Well don’t worry because you aren’t alone. The reason this happens is because the drive needs to be prepared for use, or formatted for your computer. Some space is taken by a file system that makes it possible for you to enter a file name into your word processor and have the entire file loaded and ready for use. This allows your drive to communicate with your operating system

You are actually getting the full capacity you paid for, but it is RAW space. Once a file system type (FAT32, NTFS, etc) is defined, then the drive uses a small of amount of memory to set up protocols for consumer use.

That used space is not directly accessible through normal system access methods so it isn't shown. This is to prevent the user from accidentally erasing it or modifying it. Both actions would render the drive unusable (a reformat of the drive will make it usable again) and any data on the drive would be lost.