It depends how you take it. Stress is not an absolute value like the HRV is. Stress is always relative to the baseline, which is adapting to your long-term stress levels continuously. Hence, from the point of view of absolute values, it is in fact never comparable (never the same absolute HRV baseline), while from the relative point of view it is always comparable (the percentage is always calculated in the same way from the baseline).
Of course, after the first setup, or after a reset, the values may be quite inaccurate until the baseline is really well established with a sufficient amount of data
Why is the baseline data not taken from the data already stored in Garmin Connect?
Because the developers decided so. We can only speculate about their reasons, but one of them might be that the values may differ at different devices. Another reason may be, that it in fact allows to start anew, in case the baseline was poisoned by faulty data.
Besides that, the baseline data is quite likely not even stored in Garmin Connect, and is only available on the device. It is the device who calculates the stress levels, it does not happen in Garmin Connect.
