This range chart is a little different from the others as the datasets are organised a little differently. The first dataset that you give to the Line chart becomes the bottom dataset and is an absolute set of values - in the example here the first number is 4.
The second dataset, which is the top line, is additive/accumulative. In this case the first point of the second dataset is 1. So added to the first point of the first dataset (which was 4) you get a total of 5 - which is the figure that is shown on the chart.
The reason for this is because the spline range chart is really a stacked spline chart - but with the bottom dataset using transparent as its color - so you can't see it.
This is shown below by gradually changing the color of the "bottom" dataset from blue to transparent.