If you were to truly do a science of people would you not take into account all of the circumstances that person was in, in order to understand them?
You say: "One achieved it, but the other person in similar circumstances didn't achieve it"
Well how do their circumstances differ? Don't you think it's important how they differ? Actually, couldn't how they differ be the key?
Why, then, do you draw the line at an incomplete analysis? Maybe because it is convenient? Maybe because we'd rather not destroy our illusions of ourselves? Maybe its convenient not to understand others?
What is real in regards to ones self and others? There shouldn't be a loss of pride with understanding.
What level of analysis would you consider "complete"? Certainly if we accounted for every neuron in their brain we could reduce their achievements to whatever configuration of gray matter produced the thoughts and actions that led to their success, and whatever external events produced that configuration. But then we would be at a level of analysis that regards us all as automatons, where nobody, including the group, is accountable for anything at all. This may or may not be technically correct, but I would argue that it is not useful. The question of who gets "credit" for an achievement would be entirely moot, as would the achievement itself and everything else any human has ever done.
I would think the correct level of analysis for this conversation is the lowest one that still allows people to be accountable for their own actions. Lower than that, and the central question of this thread is irrelevant.