Let's get one thing clear: It's all about the bum. Sure, built biceps fill out a shirt and six-pack abs are the prize of every beachgoer, but the back is where it's at. A bodacious booty is essential to a good physique—and not just for stage-bound fitness contestants. Everyone seems to want a great bum. Photos of posteriors flood the Internet and are often the most viewed—and "liked"—body part on social media. There's just something magical about a beautiful butt!
More specifically, you can expect to end up in the upper half of these ranges ONLY if you are a beginner, younger, and/or have amazing genetics. You can expect to end up in the lower half of these ranges if you are an intermediate or advanced trainee, older, and/or have poor genetics. The average person can expect to end up somewhere in the middle. Additional details here: How Much Muscle Can You Gain?
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.