Jim O’Malley pointed something out to me a number of years ago which maybe should have been self-evident, but I had never thought about before. He told me that every weight required a slightly different body position, and a slightly different coordination of the muscles in order for the bar/body unit to be in the correct balance and for the maximum force in the correct direction to be imparted to the bar. He said that because of this, it took greater skill to lift a series of repetitions or sets where the weight on the bar was different every time than it took to lift the same weight over and over. Of course, this is a skill that is needed for a weightlifting competition, where the weight must change each attempt unless you miss!
Over the years I have seen the truth of this, and have used this idea more and more when I program training. I use it to differentiate the training of the novice, intermediate, and advanced lifters, and I use it within the preparation period for a contest with each level of lifter.