Storage pool full warning but volume has plenty of space

Question

I have been running a storage pool of 4 disks in mirror configuration for a few years on server 2016. Today I saw a notification that the storage pool is almost full. I have attached a screen grab.

It shows that there is 923GB free in the volume, but only 9.5GB free in the Storage Pool. Looking at the health of the disks, it says the 4 disks have 0GB,8GB, 768MB, and 768MB free.

When I created the storage pool, i told it to use all the space to create the volume. Is this warning just a reflection of asking to use all the disk space to make the storage pool ? If so, why am i getting a warning all these years later ?

Should I use the "optimise storage pool" command to rebalance this very small disk imbalance ? If so, is it unwise to create a storage pool that brims the 4 disks ?

storagepool.png