different frequency domain metrics for HRV

I am attempting to find frequency domain values for heart rate variability across several packages in Python (hrv-analysis, pyHRV, Neurokit2, RapidHRV, Systole, etc.). All of my linear domain metrics are coming out the same, but not frequency domain. Does anyone know why this is? Does each package have a different implementation method that I'm not aware of? Thanks! 