The various rep ranges for the goals of endurance, strength, and hypertrophy have been fairly well defined through research and testing. But there are some individual specific variations as well. Not everyone responds in the same way to the same number of repetitions. And the number of reps for a given weight should be the number of complete reps performed with proper technique safely to fatigue/voluntary maximal action. And all of the above information does not apply to the many muscles that are secondary/stabilizing in nature in the same manner.
This is one of the best reasons to hire a personal trainer/fitness instructor. A professional with the ability to guide clients to finding the training program that works best for each individual will yield better results faster and prevents wasting time.