I know it is pedantic, but in this specific case, there is no padding :)
He/she is wasting storage because of pool geomerty.
With 4 drives, we get a stripe 4 drives wide.
Each stripe has three 4k data sectors (12k) and one 4k parity sector.
For a volblock of 16k, we need 1.33 stripes (16k/12k)...