This is actually an unfortuante trap that you fall into whenever you buy a hard dive or flash memory (and by extension, any device that uses HD or flash memory).
Mass storage makers count their bytes in thousands. 1GB = 1000MB = 1 million KB = 1 billion bytes.
Operating systems and computers, however, calculate using base 2. So 1GB = 1024 MB = 1,048,576 KB = 1,073,741,824 bytes.
See the discrepancy? So, as far as the memory supplier is concerned, they delviered to Apple Flash memory that can store 16,000,000,000 bytes of memory, or 16 of "their" gigabytes. But, to a computer, that doesn't equal the 17,179,869,184 bytes it expects will make up what's we've come to know as 16GB. Hence, that's part of the discrepancy. It's not just an Apple problem either. It's a very widespread issue.
Note that this is different from actual RAM memory. With RAM, a gigabyte really is a gigabyte no matter who's counting (because everyone in that industry agrees on what makes a gigabyte). But you can't use that type of memory on a cell phone or mobile device, because it uses much more power to stay active, and you'd lose everything the moment the battery went dead.
Don't feel bad though. Save it for when you buy what you think is a 1TB hard drive. Only to find that once you format it, you're actually "missing" a whole 92.61GB because of the same difference in opinion over what makes a terabyte.