formatting might account for a small portion of the discrepancy. the larger one (and more blatant as the drive size gets larger) is the definition and difference between GB and GiB. it's a 2.4% difference for each 'block' of 1000. for kilo it's 1.024, for mega it's 1.048, for giga it's 1.074, and for tera it's 1.099. so basically, a 4TB drive is 10% smaller than expected, and a 256GB SSD is 7.4% smaller than expected. plus or minus a bit for formatting and the inherent inability for anyone to make a drive that's PRECISELY 4,000,000,000,000 bytes or 256,000,000,000 bytes.