a) Aggregate size is shown base 1024, disk size shown is mixed based 1024/1000 (it is 1024 * 1000 * 1000). The usable size of 1TB disk is 807GiB.
b) 10% of aggregate space is reserved for internal use
c) By default 5% of remaining space is reserved by snapshots (can be set to 0%)
If you compute the result, you get exactly 13109GiB.