Capacity stated in Mac OS X or iOS
When you view the storage capacity of your Mac (Mac OS X v10.5 or earlier), iPod, iPhone, iPad, or other electronic devices within its operating system, the capacity is reported using the the binary system (base 2) of measurement. In binary, 1 GB is calculated as 1,073,741,824 bytes. This difference in how the decimal and binary numeral systems measure a GB is what causes a 32 GB storage device to appear as about 28 GB when detailed by its operating system, even though the storage device still has 32 billion bytes, as reported. You will see this difference if you look at how your computer summarizes the capacity of the computers storage, or of your iPod, iPad, or iPhones storage when the device is connected to your computer. You will also see this difference in the "About" menu on your iPod, iPad, or iPhone. The important point to understand is that the available storage capacity is the same no matter which system (decimal or binary) is used. Nothing is missing.
The storage media in your Apple product, like all storage devices, uses some of its capacity for formatting, so actual capacity available for applications and files will be less. In addition, other factors, such as pre-installed systems or other software and media, will also use part of the available storage capacity.