I may have to backtrack my observation that Suunto used to use GNSS distance alone to calculate activity distance. I just went back and looked at some older “Running” and “Walk” activities (using the new & improved Quantified Self) and noticed that sometimes activity distance was actually shorter than GNSS distance, even for older models like the SV1, 9PP, and even 9P. Not by much, perhaps only 20-40m over 5km in certain cases, but something nonetheless.
I’m mostly a trail runner, though. For these activities, GNSS distance and activity distance almost always align or activity distance is slightly longer due to elevation gain/loss.
Perhaps all Suunto watches do share the same distance algorithm (which makes sense), but the newer models are registering less-accurate GNSS accuracy and therefore applying more distance correction?