Total size of drive vs. total usable space.

I've noticed that there always seems to be less space available than what is listed for the size of a hard drive.

 

For instance I have a hard drive that is designated drive C and when I look at it's properties it reports a nice rounded number of 127GB (I would understand this to be 127GigaBytes) however just to the left of that nice clean and easy to undersatnd number is a large number break of bytes that lists it as 137,427,943,424 bytes (again I understand this to be 137Gigabytes).

 

My question is why are there two different total storage capacities listed both on the same line under: Capacity?

 

Furthermore the drive is partitioned x2. If I add up the clean number of both drives I come up missing about 12GB but if I add up the long bytes numbers of both drives it adds up to what I would expect for the listed 160 gigabyte drive.