Why fdisk disk size use 1000 not 1024 to calculate?

Question

Possible Duplicate:
Why is my storage partition xxGB, when Disk Utility, and Gparted say otherwise?

I found that 429496729600 bytes change to 429.5 GB is calculated by 1000.

But Units = sectors of 1 * 512 = 512 bytes.

Why 429.5 GB not calculated by 1024?

And 255*63*52216=838850040 != 838860800.

What happened? Why they have little difference?

My fdisk info:

  sam@sam:~$ sudo fdisk -l /dev/sda
  [sudo] password for sam: 

  Disk /dev/sda: 429.5 GB, 429496729600 bytes
  255 heads, 63 sectors/track, 52216 cylinders, total 838860800 sectors
  Units = sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disk identifier: 0x000787ed

     Device Boot      Start         End      Blocks   Id  System
  /dev/sda1   *        2048   836763647   418380800   83  Linux
  /dev/sda2       836765694   838858751     1046529    5  Extended
  /dev/sda5       836765696   838858751     1046528   82  Linux swap / Solaris
  sam@sam:~$                  

Thank you~