This was my graph, so I modified it...anyone who has ever had a jacket or boots issued to them in situations where supply levels are less than optimal knows the truth of the statement: The military operates on averages.
A military planner doesn't design a task that only 20% of males between the ages of 18 and 45 can do, they design it so that everyone in that MOS can perform it, and then they make sure people are up to the task.
For example, they know that if they make a 15 mile march with 80 lb packs, guys who get into that specific specialty that needs to do those marches will have the physical capability to do it. They also know that if they make it a 25 mile march with 80 lb packs, the percentage of men who can be counted on to complete it is going to drop.
If you are in an artillery unit, they probably figure a man can walk one 105 mm shell twenty yards in one minute and perform that task repeatedly over some interval of time. (I am making up all of this, I don't know the demands or requirements, just using these as examples.)
So they try not to make things impossible...for the greatest number of soldiers, sailors, or Marines.