These basically show similar things to zpool status and zfs list. RAIDZ has padding overhead and the GUI also mixes different units (TB vs TiB) and that's where the discrepancy comes from.
This was talked about here:
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Hello, I'm hoping someone more knowledgeable than me can help me understand how much space I actually have left on my Proxmox ZFS pool.
If I do a zpool list -v
I get an ouput saying my pool is 3.62T in size with 2.16T allocated and 1.47T free...