DPlot

I am not sure to understand the "Volume Under Surface command " and the Help file. The help file for that command show a graph. Two result windows are shown for the same graph. Since it is the same curve, the total volume (above + below) should be the same irrespective of the specified Z. Is it not?
Thank you in advance for any hint, as the Help is very succint.

Since it is the same curve, the total volume (above + below) should be the same irrespective of the specified Z. Is it not?

The short answer is "no". Imagine if the reference Z was, instead of -1, -100. The volume below will still be 0, The volume above is a large(r) number, which is correct.

It might help you to visualize this operation with a 2D curve and integrating it. The sum of the areas above and below some reference Y will NOT be the same for different reference Y's.