about ALLOCATED and FREE space

Hi all,

I am new to freenas and there are some things which confuses me.
I have 20x3T disks and two volumes (one a child of the other) on FreeNAS-9.2.1.6 :


There are two things i need your advice/guide:
1) I get "above 80%" alerts and not sure why because i only copied 10 Ts (25% full).
2) Why is there an 8T free in pool list?

I know it is probably bad setup due to my lack of knowledge but i will appreciate if you advise me on what to...

about ALLOCATED and FREE space