That's a different type of beat. In AFib you have a quivering atrium that fails to eject most of its content into the ventricle. So, the ventricle doesn't pump much blood into the system. There is a milder form of AFib which is called atrial flutter. Maybe the word flutter illustrates this best.

An exercise heartbeat is like the resting heartbeat, but faster. The electrical stimulus travels along its normal path and forces the atrium to ejct its content into the ventricle, which will then eject most of its content into the aorta.