We all differ greatly from one another and that appears to be the reason why different people have different speed of progress. There are some unique persons who gain muscle mass very easy. Rather often they are not serious about their trainings and nevertheless they succeed without even slightest understanding of with how much nature endowed them. Almost all professional bodybuilders are in this group; but the majority of ordinary people (like we are) have to work hard to succeed.
Genetically gifted professionals can workout according to any system or technique; they will grow anyway. That is why the majority of modern bodybuilding workout systems are composed exactly for such lucky people. Professionals can make whatever mistakes and still their muscles will go on growing.
An ordinary person who is not endowed with extraordinary genetics has to take absolutely all training factors into consideration in order to achieve the goals of muscle bulding and body perfection; and the quantity of these factors for an ordinary person is also higher than for a professional.