Answer:
Because they're two different people
Step-by-step explanation:
The main and major reason why, is the obvious one. The first reason that comes to the mind.
Because they are two different people. Different people have different goals, hopes and aspirations. And even those that share them go through different paths while carrying them out. The minds of the two individuals are not the same, and despite the fact that they have similar abilities, they might not necessarily get the job done on time because of other commitments either of them have. Even if they both have none, there's always a distinguishing factor, and that will always trump, thus making them have different expectancies for performing at a high level.