Okay, this one's been asked enough times that I need a FAQ entry on it.
Before I do, I want to get some exact numbers. Who can tell me the exact size taken up by each of the player's non-music partitions?
To answer your question more generally, there are two reasons:
1) Manufacturers lie about their drive sizes, pretending that a gigabyte is 1,000,000,000 bytes. Actually, a gigabyte is 1,073,741,824 bytes (or something like that, someone correct me if I'm wrong).
2) Some of the disk space is taken up by the player software and its temporary scratch partition (which it uses to store things like EQ settings and the playlist running order).