Pool space inconsistency

[Jon Schneider]

zpool list says

NAME    SIZE  ALLOC   FREE  CKPOINT  EXPANDSZ   FRAG    CAP  DEDUP 
HEALTH  ALTROOT
zroot  1.81T  1.68T   137G        -         -    45%    92%  1.00x 
ONLINE  -

whereas zfs list (and df -h) claim 79GB available.

NAME                 USED  AVAIL  REFER  MOUNTPOINT
zroot               1.68T  78.9G    88K  /zroot

How does the discrepancy between 137GB and 79GB come about ?

Perfectly default dataset arrangements with no reservations on 11.2 .

Jon