This has to do with the way nearly every hard drive manufacturer in existence calculates hard drive size. They all define 1 gigabyte = 1,000,000,000 bytes instead of the 1 gigabyte = 1,073,741,824 bytes which it *really* is.
This is called "binary" vs "decimal" sizes. If you look at the *fine print* you will always see "[Company X] defines 1 gigabyte as 1 billion bytes". This is standard industry practice, unfortunately were a drive manufacturer (Western Digital for example) to be *honest* about this then their drives as advertised would all appear smaller than the competitors, when in fact they would not be. Shoppers would be comparing "apples to oranges" rather than "apples to apples".
Put another way in reality your hard drive is 6400 million bytes rather than 6400 megabytes. A 6.4 GB drive is actually closer to a 6 GB drive when viewed from a *real world* standpoint.