zfs: df and zpool list report different size

Hi list,

I update one machine to -CURRENT (yesterday), and now I'm creating zfs
filesystem using the following devices:

ad9: 305245MB <Seagate ST3320620AS 3.AAE> at ata4-slave SATA150
ad11: 305245MB <Seagate ST3320620AS 3.AAE> at ata5-slave SATA150

Next I created the pool:

# zpool create backup raidz ad9 ad11
# mount
/dev/ad8s1a on / (ufs, local)
devfs on /dev (devfs, local)
backup on /backup (zfs, local)

# df -h
Filesystem     Size    Used   Avail Capacity  Mounted on
/dev/ad8s1a     72G    2.2G     64G     3%    /
devfs          1.0K    1.0K      0B   100%    /dev
backup         293G      0B    293G     0%    /backup

# zpool list
NAME                    SIZE    USED   AVAIL    CAP  HEALTH     ALTROOT
backup                  596G    222K    596G     0%  ONLINE     -



My doubt is why zpool list and df -h report different size ? Which of then
is correct and should I trust  ?

Any comments ?

Thank you,
Alexandre