Can you please explain to me why the body works differently for people when trying to increase strength and size. I know a lot has to do with genetics, but there has to be a lot more detail behind it all. Does it have anything to do with fast and slow twitch muscle fibres?
Genetics play a huge role in how much hypertrophy (muscle growth) a person can maximally handle as well as how strong they become. It is important to note strength and hypertrophy are two DIFFERENT goals and should be trained using different approaches. A quick answer to your question lies in the fact many clinicians, trainers, performance experts, etc. rely on educational data that is specific to one population. Health and fitness professional usually specialize in an area of performance and therefore must seek educational guidance from various experts for optimal improvement in the clients genetic potential.
Bottom line: You'll never know you reached your genetic potential unless you have exhausted all educational avenues related to the individuals goals, needs, wants and abilities. Always search for more information and never forget: "Information without a system is chaos." Develop a belief in your client, condition that belief and always use a systematic training approach.