You do realize that people even with identical genes express different phenotypes, right? That identical twins have different traits (finger prints, facial structures, etc) despite having the exact same genetic code? What makes you think muscle mass is exempt from this?
There are so many genetic factors in bodybuilding and powerlifting they are beyond naming. Muscle fiber type distribution, natural levels of test/gh/thyroid/etc, muscle insertions, anthropometric variables, and so on..
The vast majority of the population is equipped to build a decent physique and beyond normal strength levels, and way too many people hide behind their "bad genetics", but to say that we all have the same genetic potential is just delusional. We all can build admirable physiques.. but very few people in the world have potential to be a future Mr. Olympia.