Everything in computing is done with powers of 2. For example, 1K byte is not 1000 bytes; it's 1024. Likewise 1MB is 1024*1024 bytes, or 1,048,576 bytes. And 1GB is 1,073,741,824 bytes. And 30GB is therefore 32,212,254,720 bytes.
But Apple, as well as disk and memory vendors, don't sell you computer bytes. They sell you "normal" bytes. They sell you a 1KB of data that is really 1000 bytes. That 30GB iPod is really 30,000,000,000 bytes. They do this because the number is bigger and therefore more impressive.
But when your computer looks at that 30GB, it treats 1KB as 1024. So if you divide 30,000,000,000 by 1024
10241024 you get 28GB.