As zpool list always given the wrong free space ?

Just notice that my 4TB , 3 + 1 , which should be about 15TB total, but 11.5TB usable .
So with 4.91TB used I have 9.4TB free .
Mirrors look to be correct but not my raidz1-0
root@pve1:~# zpool list
NAME SIZE ALLOC FREE CKPOINT EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
SSD_Backup 3.62T 3.45T 181G - - 0% 95% 1.00x ONLINE -
master_pool 14.5T 4.91T 9.64T - - 0% 33% 1.00x ONLINE -
root@pve1:~#
as this always...

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