All this made me think about the reasons why, and since I'm a simple person, my thoughts are those of a layman...
So, one thing I've learned is that heat is the enemy of many electrical components like capacitors. Another thing I've learned is that a system can get accustomed to a higher temperature and fail if it gets cooler all of the sudden. And I know that there's a fan speed controller in power supplies to control the cooling. And that bearings of rotating things such as fans have an optimal temperature where the lubrication works at its best to reduce friction and smooth rattling.
Now if only a small percentage of power is taken from the PSU, it supposedly runs relatively cool, the fan spinning only occasionally. Not only would that keep the bearings cold, also the variation in temperature would vary between the thresholds where the fan starts and stops. Based on the threshold levels for processors in the bios settings of motherboards the difference between hot and cold can be several tens of degrees. Wouldn't that cause variations in the output as well?
Likewise, if the PSU runs at a high percentage level, the fan would keep spinning to keep the temperature at an optimal level without major highs and lows, which again would reflect to the output.
I may be speaking about the egg rather than the hen, or I may be totally off the track here. As I said, this was just some logical thinking based on limited knowledge and no education. Please correct me if needed.