Don’t quote me on this, but my understanding is that your body is very good at conserving energy, and so when you repeat a skill over and over (such as a particular weight lift), your body adapts first by using as little energy as it needs to to accomplish the task.
This might be done by decreasing the number of motor units involved, but maybe this applies more to endurance training than max strength training, I am not sure.
Hopefully someone else can help.