Apple just doesn't seem to think my walks are brisk enough to record. I guess I better pick up the pace, as I don't think Apple is going to change their algorithms based on my disappointed feedback... hahaha!
Not in this lifetime, anyway! But in fairness, I think their methodology incorporates your unique personal data and maybe active vs resting calories and what it sees about your workout progress into determining what meets the 'exercise' standard.
Whether FitBit, Garmin, or Activity, my main consideration is consistency. My idea of a brisk walk isn't what most people would consider 'brisk', but the Watch gives me nearly full credit every time. When I slow, there's a drop in credit given. So for me, the Watch and Activity app are close enough for Govt. work.