Assumes Correlation and Causation, ignores Social and Economic situations of the twins groups, doesn't account for the fact that identical twins tend to share (being geared towards) more experiences due to being perceived almost as a single "being", ignores cultural differences in upbringing of different-gender twins, doesn't account for indirect training (i.e. strategy, memorization, sequential steps etc. are parts…
No everyone doesn't. Some people are born sick, some have better cardio, some take more weight easily, some people are born twice as big, some grow very little...
And it's the same with mind abilities. Your mind have various qualities, and you are not born equals to others. Some can concentrate better, some can abstract less, some more creative, some are shy.
Add all the differences, and yes, you have people that will have an easier time than others. Bolt and Tyson have been trained, but they started with a nice base to work on.