I agree that a mental component has to be considered. With most identical twins they certainly have a distinct personality and at least some unique experiences. This impact their decision making process which like said is part of kicking. So even if physiologically they have the same capabilities, their ability to manifest them in a performance is likely different.
Also you aren't controlling for other things such as rest, nutrition, etc. Are all these things equal for the twins? Furthermore you have natural randomness. For example, suppose one twin by chance rolled their ankle which limited their training. Within a small time frame randomness can make things be observed that cannot be accounted for especially when you're talking about n=1 example.
This also brings up the point about training mental part of running. You always hear about optimal training and presumably it's talking about physiological benefits but if running is heavily mental then at what point does the mental component need to be considered when creating a training plan for an individual?