My opinion- and it is only an opinion not fact, is that everyone has a genetically gifted muscle group. Some even have two or three. Pros I believe are hyper responders to steroids plus have almost all muscle groups with genetic blessings.
Billions of people out there and if the stars and moon align….. congratulations! Even the 80’s and 90’s mass monsters didn’t take all that much and got a shit ton of results. My friend used to say “you just look at a bottle of testosterone and your back grows”. Chest, legs and hamstrings pfft crap regardless of how hard I train, eat or AAS. 500mg/wk gets me lots. Total test and deca 1,400mg/wk gets me tons and feel like shit and crash hard within months.
Henderson Thorn said to us in a small group after a seminar in 1993- “If you can’t grow like hell off 1 gram per week this is not the sport for you”