ZFSSA data profile vs allocation result

Hello Gurus,

When we configured a ZS3-2 dual-head storage appliance with Double Parity data profile, NSPF=yes

It explicitly indicate the spool capacity is 465T.

Data           465T

Reserve     165T

Spare          57.2T

However, the result is out of expectation. There is only 439T data available and 26T goes to reserve space.

Actual Allocation Result

Data             439T

Reserve      191T

Spare            57.2T

Very appreciate you could explain this situation.