Based on the way the info box follows the mouse, I think it's because their graphing tool puts an equal amount of space between data points.
Most likely, the data points they fed into it have x-values of [5%, 10%, ..., 75%, 80%, 81%, ...]. This is a useful way of tabulating percentile data, though not such a useful way of graphing it.
Most likely, the data points they fed into it have x-values of [5%, 10%, ..., 75%, 80%, 81%, ...]. This is a useful way of tabulating percentile data, though not such a useful way of graphing it.