You actually do get the full 64gb...if you paid attention enough. But it's a matter of the discrepancy between binary numbering and decimal numbering schemes used.
If you do a right click and hit "properties" of your hard drives (where you see the pie chart), and look under "capacity", you'll see a value that hits what you're looking for, if not close. Be it 64gb or 320gb.
My laptop drive is 120GB and shows: 119787220992 bytes (read, 119 billion, just a hair below 120), for a capacity of 111GB. You're not losing 9gb, it's just information being represented differently.