Hey there mate :)
Even if two persons are given the same work load, the speed of the work done gets different by the energy of those persons.
No one is sure that he/she can complete the work within the time. He may or may not.
Also, the physical characteristics makes the work different. If one person has so much power to work all day, the other person may not have.
Therefore, even if two persons do the same amount of work , they may have different power.