Thank you for the opportunity to help you with your question!
The reason why its not possible to have an accurate measure of potential income is because the sources of these income are not fixed and the amount keeps on varying. Their outcome is unpredictable since they depend on natural factors as well as man made factors which can not be predefined or determined. There is uncertainty in the factors of production which lead to the income.
Please let me know if you need any clarification. I'm always happy to answer your questions.