Our HRV score is calculated from the RMSSD and represents the strength of your Autonomic Nervous System (specifically the Parasympathetic branch) at a given time. The other numbers provided are for reference for those interested in in more detailed analysis of their HRV data.
Research standardized calculations such as RMSSD can still appear slightly different between systems. This could be due to differences in hardware (heart rate monitors, etc.). But it is usually due to differences in the way misreads and signal noise are handled.