Why does the amount of overload vary from indivdual to indivdual?

Some different people may have to work more on flexibility, whereas I may have to work harder on muscular endurance. The amount of overload needed by different individuals can easily be seen when comparing a younger person and an older person. For example, If my grandma has been inactive for some time, she may find a fast walk stressful, while I may have to jog for 2 miles to achieve an overload.