Technically a gigabyte is 1000 megabyte. Megabytes are 1000 kilobytes and kilobytes are 1000 bytes. Which are all proper metric units but sadly don't make any sense. So datasystem manufactures and computer generally calculate with their proper counterparts that you mentions gibibyte mebibyte and kibibyte which sre actualy 1024 of their previous ones. Small but crucial difference.