Nominal capacity, also know as the "advertised" capacity, is based on the standard base 10 numbers instead of the base 2 mathematics that disk drives use.  In bae 10 one Megabyte equals one million bytes while one Gigabyte equals one billion bytes. This isn't too much of a problem with small numbers such as a Kilobyte, but each level of increase in the prefix also increased the total discrepancy of the actual capacity compared to the nominal capacity.

Check out the table below which has some rough estimates of the formatted capacity of standard disk drives.