Good question. Each key in a deform timeline stores the position of all mesh vertices. The graph normally shows the value for each key, but it wouldn't make sense to show the value of all mesh vertices for each key. Instead, the graph shows the percentage of difference between one frame and the next (similar to the Curves view). This means the curve in graph between two deform keys always starts at 0% (the first key) and ends at 100% (the next key).
