Overview
You really DO have the space you were expecting, minus a couple of MB for overhead, out of all that TB of space, plus some space for the filesystem itself.
it's not actually as much as you might think (unless you've got a really full disk and you have a lot of little files, then the $MFT part of the filesystem takes up fair chunk).
in the usual case, however, with a new disk, what you are seeing is a microsoft-ism. it's one of the two ways of looking at sizes of disk space. 1024 bytes for 1K, or 1000 bytes for 1K. microsoft chose 1024 bytes for 1K. but microsoft doesn't say it properly - it's properly said 1KiB=1024 bytes. so you see less disk space when you look at the disk size numbers it reports. at least it's consistent.
calculations
so, for a 750GB drive, you will see 698GB instead. here's how:
750e9B÷10243 = 698.491930962GiB
1GB = 10e9B
1GiB = 230B = 10243B = 1,048,576B×1,024B = 1,073,741,824B
industry measurement usage
when you buy a hard drive, it is measured in GB. when you buy RAM, it is measured in GiB. Microsoft made a mistake in measuring disk space in MiB and GiB. and they won't quit either for some reason.
- Hard Disk Industry: properly 1kB=1000 bytes using 512 bytes and (soon) larger sectors, but they say 1KB, 1GB, 1TB. SI units should properly be in lower case. but when used with computers, they are used in upper case for some reason. doesn't mean anything. so I guess it's a defacto standard.
- Optical Drive Industry: properly 1KiB=1024 bytes using 2KiB sectors, but they say 1KB, 1MB, 1GB. this is a "computer unit of measurement".
- RAM Industry: properly 1KiB=1024 bytes, but they say 1KB, 1MB, 1GB. this is a "computer unit of measurement".
- SSD Industry: typically IEC units, 1GiB=1024^3
- Windows: numbers are IEC units, but labeled units are SI units (confused and improper)."1GB"=1024^3: very wrong, defacto standard bug ands much debated.