This is how Seagate explains it: Discrepancy Between Reported Capacity and
Actual Capacity
Many customers are confused when their operating system reports, for
example, that their new ST310240A 10.24-Gbyte hard drive is reporting only
9.85 Gbytes in usable capacity. Several factors may come into play when you
see the reported capacity of a disc drive. Unfortunately there are two
different number systems which are used to express units of storage
capacity; binary, which says that a kilobyte is equal to 1024 bytes, and
decimal, which says that a kilobyte is equal to 1000 bytes. The storage
industry standard is to display capacity in decimal. Even though in binary
you have more bytes, the decimal representation of a Gbyte shows greater
capacity. In order to accurately understand the true capacity of your disc
drive, you need to know which base unit of measure (binary or decimal) is
being used to represent capacity. Another factor that can cause
misrepresentation of the size of a disc drive is BIOS limitations. Many
older BIOS are limited in the number of cylinders they can support.