Originally posted by MrBubbles
I think the issue is really with the definition of gigabyte. It can be either 1 billion bytes exactly (which is probably what the manufacturer reports) or it can be 1073741824 bytes (1024^3 bytes; which is probably what the iPod itself and iTunes report). The second definition is the traditional one used by computer scientists and friends because it's a nice power of 2, the first is used by PR folks because it's a nice power of 10 and because it inflates the numbers.
If you still don't get it, consider that 15,000,000,000 bytes (15 PR GB) divided by 1,073,741,824 is approx. 13.97 traditional GB -- not quite 15GB.