It's complicated, but https://jro.io/capacity/ actually walks you though how it works, towards the middle of the page.

``` Unlike traditional RAID5 and RAID6 implementations, ZFS supports partial-stripe writes. This has a number of important advantages but also presents some implications for space calculation that we'll need to consider. Supporting partial stripe writes means that in our 7wZ2 vdev example, we can support a write of 12 total sectors even though 12 is not an even multiple of our stripe width (7). 12 is evenly divisible by +1 (3 in this case), so we don't even need any padding. We would have a single full stripe of 7 sectors (2 parity sectors plus 5 data sectors) followed by a partial stripe with 2 parity sectors and 3 data sectors. This will be important because even though we can support partial stripe writes, every stripe (including those partial stripes) need a full set of p parity sectors. ```