advertised sizes are basically shown to be more than the genuine available space. since the OS works it out in binary math, the number of bytes to calculate a gig is lower.
so the drive has 640gb when caculated in base ten math, but only 596 when calculated in binary math.
plsu, a small portion of a HDD is used for firmware or random software that you cannot access. this is only a fraction though, the biggest thing is the deffierent math used to work it out.