Muscle maturity, knowledge about what your are doing.... and the only way to gain knowledge and tricks of the trade is be around the sport for time...
I also think most start using compounds similar to how bodybuilders do...they have to start on low doses and build up overtime. .. Just because you are on 10 grams of gear, and 20ius of hgh, doesn't mean you grow into Phil heath over night...
It takes years of training to develop the muscle memory and technique these guys use to push those monster amounts of weights....
Look at some of the ronnie coleman videos around the time he was being uncrowned by cutler, that fuck was doing 800lb squats and benching 220lb dumbells, and that was towards the end if his career....
I think given the gear these guys run, you will never stop making gains, until injury happens and the body can't take anymore...