When someone says the deltas are zero, it means when the camera stops shaking, it goes back to rest at its exact original position.
In an ideal condition where the camera is perfectly balanced and there's no external force acting upon it (like wind), then after any disturbance, it might return to its original position, and the mean of the changes in its position (the deltas) might be very close to zero. However, real-world conditions often introduce factors that can cause a deviation.
If a camera is mounted on the wing of a flying MQ-9 Reaper drone and you're measuring the "deltas" (changes in position) of the camera as it's mounted on the wing of a flying drone, they will likely vary significantly.
The mean of these deltas would probably not be close to zero, especially if the drone is maneuvering or facing turbulent conditions.