The numbers are multiples of two. This is why a kilobyte of memory is 1024 bytes and not 1000 as would be the case for kilogram. To try to avoid this confusion the International Electrotechnical Commission (IEC) use the names kibibyte, mebibyte, and gibibyte for binary powers. They use kilobyte, megabyte, and gigabyte to mean powers of 10. The Joint Electron Device Engineering Council (JEDEC) have kept the older names. To make it worse the sizes of computer storage, such as hard disk drives (HDD), are measured in powers of ten. So a 500GB disk drive is 500 x 1000 x 1000 x 1000 bytes. This is a lot less that 500GB of memory which is 500 x 1024 x 1024 x1024. Most computer scientists still use the old names and have to remember that the units are different when talking about memory and storage devices.