I'm not sure if this is just a quirk of how ZFS ZPL is presenting the available space, but it seems that SNMP is reporting the free space no more than 2TB higher than the used space.
This is on a mount that has no quota or reservation set, just a simple
zpool create and then filling with data over time.
The total space in that pool is actually 6TB.
I wondered if maybe it could be happening because of an integer type used in my Cacti graph template.