I've been wondering about this since I've been watching my HRV scores tank over time and decided to compare the FR955's measurements to what I have been seeing from another HRV app (plus a chest strap).
While the min and max HR's are pretty close (within a beat or two) and the Average is spot on, it seems that the RMSSD and SDRR numbers are high. Maybe, they're within the margin of error? Just another data point I am throwing out there.
Elite HRV + Polar H10 strap:
FR955 HRV in Health Snapshot:
Note: both measurements taken simultaneously.