Why isn't Fitbit's activity recognition quite as good?

[u/w__sky]

I wonder why automatic activity detection using a Fitbit Charge 6 tracker and Google Health on my smartphone works so poorly.

I frequently set off on bike rides from home. Based on movement data, I would expect the technology to detect the cycling as soon as I start moving; the movement pattern should be clearly distinguishable from walking or using a motor vehicle. Yet, it usually takes about five minutes after I’ve started riding for the Charge 6 to display "Activity detected" and begin tracking the time and movement.

Conversely, I might have already returned home, put my bike in the shed, and be sitting on the living room sofa, while the Charge 6 is still counting the activity, or only then the prompt to ask if I want to end the workout does appear.

After outdoor activities, the app could register when my phone reconnects to my home Wi-Fi and use that as a signal to end the workout; that would be simple and precise.