Now that I think about it this makes sense to give a more 'definitive' answer to a customer since it is an easy to understand concept and meets the 'average' persons requirements. There would be no way or point in trying to explain the intricate differences in exercise physiology & dynamics, detailed algorithms, individual metrics or trying to explain perceived exertion (Borg) to the general public.
We do know that Exercise Minutes are credited for every full minute of movement equaling a brisk walk ("for wheelchair users, this is measured in brisk pushes"), unless using the Workout app. It's even stated in the Health app under the Exercise Minutes section.
The Apple Watch is obviously not using the GPS to determine speed. It's clearly using accelerometers and other sensors for those calculations. Therefore, Apple has to be using algorithms based on individual metrics in order to determine physical activity. For example, my 6'11" coworker will naturally be taking less steps and swinging his arms less during a 3mph walk than someone that is 4'11" walking at the same speed. More footsteps ≠ faster speed after all. The Apple Watch already uses my height and weight to determine my Active Energy and Resting Energy, why wouldn't it do the same for other calculations?
Perhaps the reason TxWatch isn't seeing the same number of exercise minutes as his wife is because his gait isn't maintained above the "exercise" threshold for a full minute. Seconds really do count, as evidenced by digging into the data in the Health app.
