Here is an approximate breakdown of what you will see on your computer for it's respective selling size.
Decimal
Size You Bought 
= 
Binary
Available Space
On Your
Computer 
100GB 

93.1GB 
320GB 

297.95GB 
500GB 

465.55GB 
640GB 

595.90GB 
750GB 

698.32GB 
1TB 

931.10GB 
1.5TB 

1396.65GB 
Hard drive manufacturers use a calculating method different than how computers calculate figures, so there will be nominal differences. This is how hard drive manufacturers have been selling drives since day one. We agree that showing accurate amount makes sense. Though selling 111.68GB hard disk probably isn't as easy to understand as 120GB.
The Technical Explanation:
Decimal vs.. Binary:
For simplicity and consistency, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal (base 10) measurement and is the industry standard. However, certain systems define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. These are binary (base 2) measurements.
This is why different utilities will report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make a megabyte and a gigabyte. This is similar to the difference between 0 degrees Celsius and 32 degrees Fahrenheit. It is the same temperature, but will be reported differently depending on the scale you are using.
