A professional follows a strict schedule of training, resting, eating, long rides, etc. Thus, he/she might count on a relatively stable heart rate that can be a base of measuring “HR - zone 2” riding.

By contrast, a casual rider performs intermittent training (usually has a job as the main activity), rests and eats less “regulated”, may train under psychological pressure of other day to day events, under variable temperature (even 38-40 Celsius – if this is the available time slot), etc. All those can easily introduce a bias of +/- 15 beats, which makes HR zones riding to be very misleading.