I have this system:
Filesystem Size Used Avail Capacity Mounted on
$ df -h
tank 885G 0B 885G 0% /tank
tank/files 885G 0B 885G 0% /tank/files
tank/files/foo 916G 31G 885G 3% /tank/files/foo
When showing @ http://grimstveit.no/admin/phpsysinfo they sum up all three mountpoints to for example 885G*3 = 2722.92GB avilable. ... Which is not correct, since free disk space is share amongst mountpoints in ZFS.
Can try to provide a patch unless anybody else wants to :-)