Update: We did another test today. Outside brisk walk using the exercise app on the watch and selecting Outdoor Walk. Phones were taken with us as well. For my walk, I got credit in the exercise ring for most of the walk, I think it was all but 4 or 5 minutes of a 40 minute walk. My wife however only got exercise credit for about 20 minutes. We verified this by ending the walk, looking at her phone which had 40 minutes of walk on the exercise and then going to the exercise ring to see if it increased by this same amount. It did not. Note that her heart rate is lower than mine and stayed below her calculated optimal exercise level almost half of the time. I have really come to the same conclusion as Jwolf74 did earlier in this thread:
"Workout activities don't seem to count either unless your heart rate reaches the target,"
It seems if you are in this target range then it counts as exercise, otherwise it does not. This could be the reason why others see different results. Now, the only thing I am not really sure of is if Apple uses the standard calculation (see below) for target range or has its own method of coming up with a target range (perhaps based on resting or average heart rate over time).
Max BPM = 220 - Age
Target Range = Max BPM * (0.70 to 0.85)