That's perfectly normal, you'll find this with all drives, my 30GB is reported as 27.88. The same is true if you look at the size reported for your own computer hard drive. For instance my 300GB hard drive is reported as 279.36.
Why the difference? Most hard disk manufacturers measure disk size this way: 1 MB = 1 million bytes (1000 * 1000). A 4 GB disk, therefore, is one that holds 4 billion bytes. Computers, including Windows computers, Macintosh computers and iPod, measure disk size this way: 1 MB = 1 048 576 bytes (1024 * 1024). The difference in these two calculations is what causes the drive to appear as 3.7 GB on a computer, but actually be a 4 billion byte disk.
You can read the whole article here:
How much content will fit on my iPod?
There is also a proportion of the space on your iPod taken up by the operating software but the difference is mostly down to how the size is measured by the manufacturers v the OS.
