Those with a strong work ethic are often dismayed to find that they are dealing with many times the workload of their less-motivated colleagues. While most people believe that this can be explained by the fundamental unfairness of the universe, I believe there is a more specific effect, "The Theory of Demonstrated Competence", that provides a better explanation.
This theory states:
As an individual displays competence in a particular task or area of knowledge in the workplace, they will become responsible for that task or knowledge even if it isn't part of their defined job description.
Often, these responsibilities are shifted from those who do not demonstrate competence even though they are part of the less-competent worker's formal role.
There are several underlying factors that contribute to this phenomenon. Since management is, generally, rewarded for the aggregate work product of their reports, they see a higher return if they shift responsibilities to those who are clearly able to fulfill them. Being able to produce more with the same resources results in greater rewards for management.
The competent individual is often driven by the lure of being considered "a team player" or by the statement that "somebody's got to get it done". These individuals generally don't see the bigger picture in which they are doing more work for the same rate of pay and, generally, go through stages where they feel burnt out by the cumulative stress. Because these individuals usually exhibit strong, Puritanical work ethics, it can be difficult for them to develop strategies to avoid demonstrating competence.
The less competent usually manage to do well in the enterprise in spite of their incompetence. Part of this is inertia -- in the corporate world today, it is very difficult to fire an individual in the absence of a specific provocation (such as a sexual harrassment charge or obvious malfeasance). Individuals who simply do not perform well would require more effort to fire than the effort required to simply shift their responsibilities to other parties. Additionally, for many of these individuals, "The Peter Principle" comes into play.