I am trying to understand how to determine the Navigation Accuracy for a given flight phase of an Instrument Flight Procedure, from the US Standard for PBN Instrument Procedure Design (Order 8260.58A). However, I am unable to understand how to determine the aforementioned accuracy under certain circumstances, given that, for example in case of A-RNP navigational specification and STAR flight phase, it states that:
1-2-4.c, Table 1-2-1 A-RNP permits a range of scalable RNP lateral navigation accuracies. Apply the largest RNP for the flight phase unless a smaller value is required to achieve a desired ground track or is operationally required. RNP APCH applies to final flight phase.
My question is, in other words, why a desired ground track would require a smaller navigation accuracy value? Under what circumstances such a ground track would be desirable and, since the goal is to provide an adequate Obstacle Clearance Area, doesn't this mean that smaller accuracy value would lead to a smaller obstacle clearance area?
It is a really technical question and I do not expect to be a popular one, but I would really appreciate if someone could point me in the right direction.