It is very easy to answer. Different people have different efficiency, therefore, a task completion does not depend solely on its complexity but also on who is

It is very easy to answer. Different people have different efficiency, therefore, a task completion does not depend solely on its complexity but also on who is executing it. You should think about it (even though there are no actual physical units you can use to make this work): Time to deliver a task = complexity / developer efficiency This is why it is hard to develop a method to estimate time for development tasks.