Here is an approximate breakdown of what you will see on your computer for it's respective selling size.
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.
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.