Why is my RAIDZ2 pool larger than the expected size calculation?
It turns out that each drive is 1.82TiB. That, times 6 = 10.9TiB. I suppose zpool reports the total physical space available for filesystem labour (ie: data + parity) as opposed to just the space available for user data without the parity factored in.
In fact, the "duplicate question" has a perfectly good answer already, quoted here (added dashes for clarity):
zpool list- shows the size of the pool, which is the size of all the disks.
zfs list- shows the usable file systems sizes in the pool.
PS. Maybe we should have a "raidz2" tag.