Difficult tasks have a steeper learning curve than simple ones when the time in which the task is to be learnt is not changed. In this situation the denominator (time) remains constant, whereas the numerator (knowledge) increases. For the learning curve to retain the same gradient, time taken for learning needs to increase in proportion to the difficulty of the task.

To me, undergoing a steep learning curve relates more to the time necessary to accomplish the task, rather than the difficulty of the task itself. Even an easy task will have a steep learning curve if it has to be completed quickly. The denominator (time) is crucial in determining the gradient of the learning curve, while the numerator (knowledge) has little influence.