To count as "exercise" the watch needs to record a certain level of heart rate and movement, and has nothing to do with whether it is recording a workout or not. You could, for example, start recording an "outdoor cycle" for an hour while sitting on your sofa or in a car, but that would register as zero minutes exercise.
I frequently go for casual/social bike rides with my wife which are not strenuous, and although it may record over an hour bike ride, it records it only as a few minutes of real "exercise."
If your bike rides are strenuous for the full duration, then the exercise minutes should be similar duration. If not, it may be because it is not reading your heart rate properly. This happens to me when mountain biking or if the watch is not snug enough on my wrist: while mountain biking the watch is often bouncing around so much it cannot get a heart reading.
Hope this helps.