Activity green ring not filling while non-workout outside walk or cycle [Apple Watch 2]

Question

After my Apple Watch 1 I decided to switch to the Apple Watch 2 and gave my GF also an Apple Watch 2. The strange thing is: My green activity ring is filling during my day without starting a workout (just walking stairs etc) but my GF's watch barley updates the green ring.

Example: We are walking in the city next to each other. We both have around 2000 steps and walked 2K without a workout. My green ring is at 40%, hers is at 3%.

How is this possible? We set the same movement goals.

Answer 1

The watch uses your heart rate to decide whether you're exercising. Maybe your heart rate increases over the threshold and hers isn't?

More info: http://www.imore.com/apple-watch-activity-tracking-5-tips-you-need-know