Converting from raid 1 to 5 has not given me the space

[Gilbertr14]

I converted my 4 TB mirror to a 4 x 6TB raid 5.

I replaced the 1 4TB with a 6 TB, let it resync, and then replaced the other 4TB with a 6TB.
I then expanded the raid to raid 5 by adding the other two disks.
This should have given me 3 x 5.xx TB = 15.9TB odd of storage, with the 4th being used for parity.
However, I only get 10.89TB which suggests that it's 3 in the raid with a spare, but this is not how is configured.

Device is TS-453A

How would I correct this?

[dolbyman]

On a static volume, yes

On a pool with thin/thick volumes, you have to tell the system what to do with the additional pool space, take a peek in the manual