The stock curve spun the fan up on every short burst of disk activity. Disks care about sustained temperature, not spikes, so the curve now follows the drives with a slow average:
below 35 C 30% 35 - 40 C 30% -> 50% 40 - 45 C 50% -> 80% above 45 C 100%
With a two-minute moving average the fan barely changes speed during the day, and the drives stay under 41 °C.