Really the author fails to come to the conclusion that you should theoretically train your muscles a different number of times throughout the year. At some points I will train the same group up to 6 times, and at others at most 1 time per week. There is no right or wrong. What we do know is that monotony leads to staleness regardless of how many times per week you are training, if it is prolonged without change in other training variables then staleness occurs.