WatchOS 3 and iOS 10 - Exercise times
WATCH determines a certain activity threshold beyond which it will consider an activity to be exercise. This threshold is specific to each individual, and takes into account your heart rate throughout the day vs. during an activity, your previous activities, etc.

Your average heart rate for that walk seems high enough to be considered exercise, although once again it is difficult to tell the exact threshold WATCH is using for your specifically. With that in mind, if we assume that the average heart rate is high enough, then that leaves us with the pace.

According to Apple, an activity needs to be at the level of a brisk walk or faster for it to grant credit. You may try checking periods where you might have walked faster or slower, and seeing if you can correlate those to the bars on the Exercise graph.

I hope this helps. I’m sorry if it can sound demoralizing to have an electronic device “judging” your activity level, but don’t get discouraged!