I thought this was just because disk makers advertise their drives with capacity in a special marketing/literal form of Gigabyte, being 1 billion bytes. Therefore making disks look bigger than they are.
But, the computer gigabyte is 1024Mb = 1024*1024Kb = 1024 * 1024 * 1024bytes = 1073741824 bytes.
So your disk actually has (72.8 billion) 72800000000 / 1073741824 bytes capacity which is 67.8Gb