This is because, for example, two scenes might have the same length, but one might cover twice the time span of the other. Now, when the user moves the slider to change the point in time span, one scene appears to be twice as fast as the other. A more sophisticated way is to select a clock and show all scenes ordered in time. Note however that not all scenes might be connected to the selected clock and thus might not show up in this view. Usually however, there is a main clock to which all scenes of the main story are anchored.
Scenes missing here are either dreams or unconnected events.