A passenger inside a train car accelerating horizontally to the right with constant acceleration \(a_0\) releases a small sphere from rest relative to the ceiling. A stationary observer on the ground outside the train watches the sphere fall through a transparent window. The observer inside the train records the trajectory of the sphere as a straight line tilted backward, whereas the observer on the ground records the trajectory as a parabola. Which of the following correctly explains why the two observers record different trajectory shapes for the falling sphere?