Disc Shrink

Hi every one,
I am trying to shrink a disc in win 7
I needed 200gb
at the prompt i put in 200gb but it gave me 195.31 after the shrink.
does any one have any idea?
or what are the caculation rules to follow in disc partition?
for example if I want to partion a HDD of 1000gb into 2
I should expect some loses, I believe it is not going to give me the exact 500gb 500gb, any one has an idea of this caculations?
Thank you for your reply.

1024bytes = 1mb
1024000 - 1gb
204800000 = 200gb

Thanks alot

Just need to remember that Data is not in 1000's it is in 1024's. just as long as you got it sorted