This is why, if you search zfs-related mailing lists, they all recommend ignoring du and df when using ZFS as they won't give you the results you expect.
Instead, use zfs list
. That will show you the correct information.
If you search the zfs-discuss mailing list archives, there's several long-posts that describe how du, df, zfs list, and zpool list interact and why they show different values. I've also made similar postings to the freebsd-fs list years ago.
Basically, the fs-related tools of the past don't work on the next-gen fs like zfs and btrfs. Stop relying on them, and use the fs-related specific tools.