It does because the motion of the other objects is embedded in their position over the previous few frames. On the contrary, it is precisely filtering on moving objects such that the velocity vector is directly on a collision path with the camera. Objects which are moving but ultimately not on a collision course will be in different locations once the perspective transform is applied and thus average out to background. Objects which are moving on a collision course relative to the observer get amplified by this simple filter (irrespective of if the motion is absolute or not relative to ground).