Diminishing share of synchronous generators in the power system demands that inverter-interfaced generation sources provide grid-forming capability and ancillary services conventionally offered by SGs. Recognizing the pivotal role of grid-forming (GFM) inverters in this context, the National Grid Electricity System Operator (NGESO) recently amended its grid code facilitating the establishment of an ancillary services market based on GFM plants. The Network frequency perturbation (NFP) plot is recommended in the amended grid code as a compliance testing tool for a GFM plant. The interpretation of the NFP plot in this grid code and associated NFP plot interpretation guide is based on the oversimplified small-signal model of GFM inverters, which only considers the low-frequency dynamics of the active power control (APC) loop. Although the resulting model and analysis is simple and intuitive with such considerations, it is not known how ignorance of the dynamic coupling between the APC and RPC loops, as well as the electromagnetic transient dynamics of GFM inverters, may affect the interpretation of the NFP plot. In order to address this challenge, the accurate small-signal model of a typical GFM inverter is established in this article, based on which the NFP plot is developed and compared with its simplified version proposed in NFP plot interpretation guide. The outcome of this work provides insights for practitioners on the frequency ranges and conditions under which the simplification of the NFP plot can or cannot be made for modelling and interpreting the NFP plot of a GFM inverter. Experimental results are provided to validate the theoretical analysis.