I know ... old topic but actually never had any issue with the system and I could not find anything wrong. Never payed attention to this detail (was running unraid 4, 5 with 1.5T disks in the past) and probably the stats were not clean when parity check was started. Anyway, I can post a picture tomorrow as I'll run a new parity check (counters reset) and probably I'll get the same, huge difference even if all disks are 3T / 4K aligned, xfs and two of them are almost identically filled and the same model (WD Red 30efrx) but different firmware.
I'll post a picture as promised, but won't dig further this night.
Thanks anyway