Why the internal hard drive and external hard drive not in the full capacity?

Tweet

I have this curious question for quite while, and I just want to find out why it is that. I notice that when I install my internal or external hard drive, for instance, 1 TB external hard drive. When I plug it in to the USB port, and I saw the property only show 990 GB or something like that, and it wasn't in its fullest capacity amount. Why is that? It is like every hard drive would do the same, but the actual capacity doesn't match what they say in the box.