That's because the bike takes the weight of the person's structure. If you have a lifestyle where you do something like chin-ups regularly, your body intuitively knows that if you eat past a certain point, you are only making it harder for yourself to lift during your next workout. With a bike your body is just trying to build as big an energy tanks as possible, it doesn't matter if it is a gut. I have a friend who has been in great shape his whole life. He loves to hike and climbs mountains all the time. The only time I ever saw him with a pot belly was when he spent a few months bicycling East to West across North America. His body was doing everything it could to create a calorie surplus and there was no negative consequence to having too much because the bike takes the weight.