Marketing vs Technical GB
In marketing terms a gigabyte is 1,000,000,000 (10^9) bytes.
In technical terms a gigabyte is 1,073,741,824 (1024^3 or 2^30) bytes.
What happened is one disk company started marking using decimal MB numbers and all the other disk companies followed. (Made the disks look bigger.)
Of course, computers still use binary and so KByte=1024 and MByte=1,048,576.