There are two reasons that the available disk space is less than you may expect:
Some of the disk space is always used to keep track of the data. A FAT file system for example uses a file allocation table to link file clusters together into files. The file allocation table is reserved for all the disk space from start, so it uses a certain small percentage of the total space.
Disk space is either calcualted using scientific prefixes or binary prefixes. A kilybyte is 1000 bytes while a kibibyte is 1024 byte. Hard drive space is always specified using scientific prefixes, so a 1 TB disk is around 1000000000000 bytes. The operating system always shows the disk space using binary prefixes, so the disk shows up as only 931 mebibyte. To further add to the confusion, most systems still use the scientific prefixes even if they calculate the numbers using binary prefixes, so the systems says 931 MB when it really means 931 MiB.