incorect data size with z3 please help

[5w1ft]

Hello

I am running the following
freenas 8.3
I have 28 x 2tb drive (1.8189894TB) ALL on 1 z3 pool
now the calculation should be as follows
(28drives - 3drives) x1.8tb = 45tb
but on my freenas it shows 39.7tb

my hardware does pick up the drives correctly

Is there any reason for this, why am I loosing 5tb

 

[Tysken]

I think there is a swap partition on each disk using 2GB as default and zfs is also reserving 1/64 of the pool size. But 5TB sounds like a lot.
I know somewhere in the zfs ninja video he talks about calculating pool sizes.
http://www.youtube.com/watch?v=3-KesLwobps