Bytes are measured using a base 2 numbering system (as opposed to the more familiar base 10).

1 Kilobyte (KB) is actually 1,024 or 2 raised to the power of 10.

1 Megabyte (MB) is actually 1,048,576 or 2 raised to the power of 20.

1 Gigabyte (GB) is actually 1,073,741,824 or 2 raised to the power of 30.

Therefore, 1,073,741,824 divided by 1,048,576 = 1,024.

(see Bits & Bytes)