Why formatted capacity of an optical disk is less than the unformatted capacity?

Answer 1

any filesystem has a space that is not user available , it is meant to be used by the os to store system data (name,block addreses etc,depending on the filesytem) about the files on the userspace.
This takes up the space "missing".
hope this helps

Answer 2

When a disk is formatted, part of the space is taken up by a directory. This keeps track of where all of your files and programs are on the disk, so they can be found later.

Answer 3

several reasons. The file allocation table takes room (FAT) which holds your filenames, folder names, etc.
Plus, don't forget that 1GB =1024 MB, and 1MB = 1024 KB, so a disk of 1000000 bytes is only really 0.95 GB, not the 1GB that the manufacturer may have stated.