Grid Forming (GFM) inverters are valuable assets for enabling sustainable power systems. Network Frequency Perturbation (NFP) plot could serve as a valuable tool for evaluating the behaviour of a GFM inverter's active power control (APC) loop. NFP plot has been recommended in the recent grid code modification by National Grid ESO as a document to be submitted by a grid forming (GFM) plant operator for compliance testing purposes. In a grid connected GFM inverter, it is well known that the APC and reactive power control (RPC) loops are inherently coupled with each other. However, state of the art guidelines on producing and interpreting an NFP plot have completely disregarded the influence of this coupling effect in an attempt to simplify the analysis. In this paper, a systematic analysis is presented which identifies the conditions under which such a simplification could be valid, or could otherwise introduce significant influence on the NFP plot. Simulation results are presented validating the theoretical analysis.