Zpool size less than dataset

Hi,

I've created a volume through the ZFS Volume Manager (zpool1) with 8 disks of WD Red 4TB in a raidz2 configuration, and then created a zfs dataset on it.

Why is the size of the volume zpool1 equivalent to the available, free space of the disks, and not 20,3 TiB as that's the maximum of the set? How can the windows-dataset be bigger than the volume it's on?

Here's an image to illustrate.
View attachment 4880

Secondly, why is the maximum size of the 6 disks (taking away two of the...

Zpool size less than dataset