What most people fail to understand is that your body adapts to averages, it can't store 2 different sets of adaptions, for example if you were to train with 3 sets of 5 reps followed by 3 sets of 15 reps your body will adapt to the average so you may as well perform 6 sets of 10 reps. Your body doesn't store a 5 rep adaption and a 15 rep adaption and switch between them at will.
It's not the magic combination of low + high reps giving you the "best of both worlds", rather it's the simple fact that your average reps per set has increased from 5 to 10 which is much more beneficial for hypertrophy.
Take a guy doing only 5 rep sets and get him to start doing 15 rep sets as well, his average rep range is now 10, he'll experience great hypertrophy. On the other hand take a guy doing only 15 rep sets and get him to start doing 5 rep sets as well, his average rep range is now 10, he'll also experience great hypertrophy.
The reason why combining low rep + high rep training works better than standalone low rep training or standalone high rep training is because it either increases or decreases your average rep range to a more moderate number which is better for hypertrophy.
6 sets of 10 reps is just as effective as 3 sets of 5 reps + 3 sets of 15 reps. The body adapts to averages. If you train with a moderate rep range to begin with you're getting the same results as someone who combines low rep + high rep training.