Here is what happens to me on lower versus higher power in the same zone. At lower power, I tend to focus on constant pedal force and my cadence drops. Why? At lower cadence it is easier for me to make accurate power. I tend to be more relaxed and my posture changes on the bike and not for the better.
As I begin to increase power, my back flattens and I have slightly less weight on my hands. My posture becomes more aggressive and I may slide forward on the saddle. More weight is now being supported by my feet.
For me, I think small changes on the bike generate different feelings of fatigue during and post ride. I do not like recovery ride that feature higher cadence and no power. They make my sit bones sore.
You may be adding more muscle tightness in your legs trying to ride easier than just relax and let it happen.