Training Efficiency Percentage (TEP) is defined as the number or percentage of reps in a given set that elicit an adaptive response for hypertrophy. If you observe a very advanced trainee and a beginner-intermediate trainee perform a set of 10 you'll notice a difference in how the muscle fatigues. For the average person, doing a set of 10, may not show difficulty in the performance until the last few reps. But in the advanced trainee, you'll notice the first and last rep appear to be of equal intensity. This is because there is a greater TEP in the advanced trainee than there is in the intermediate lifter; for the beginning/ intermediate lifter difficulty only comes near the end of a set. This is a learned response/adaptation that takes place over time.
Or to put it another way: learning to recruit larger motor units for performance is a skill set developed and refined over time. This is why proper individualized program sequencing throughout the training year and coaching is so important in development.