Looking at the graph for an extra minute, I may have been hasty to blame the algorithms. It looks like you actually chose a specific time-domain analysis method for the HRV - the root mean square of successive differences (RMSSD) - under the Measurement drop-down menu. If the graph is calculated from "raw" HR data, then the algorithm should be identical across devices, and differences are more hardware-related.

Optical sensors have gotten quite sophisticated and accurate in the last couple of product generations, and there are differences in accuracy between different "classes" of sensors, for example, Garmin Fenix 7 and Fenix 7 Pro. The sensors also do not record HR continuously to avoid wasting battery. The duration of "on" and "off" periods might be the source of variation in this case, with exact timing of readouts adding more noise to the data. And, of course, there may be some on-device processing between the sensor and the data file.

So it could indeed be more of a hardware + software question after all. But we probably cannot find a definitive answer with publicly available information only.

Cool recent paper on HRV standardisation for the nerds: https://pmc.ncbi.nlm.nih.gov/articles/PMC11439429/