Volume (the number of tough working sets per week) has been described as having a dose-response relationship with hypertrophy in the scientific literature. This means that more volume causes more growth (up to a point). If a higher training frequency allows you to perform more weekly volume, that should lead to more muscle growth (again, up to a point). After all, if you only hit your shoulders once a week, you have to cram ALL your weekly volume into a single workout. This program starts with 22 sets of shoulder work per week. Imagine trying to cram all that volume into one (or even two) workouts per week!