So I opened your report up and compared side by side. The surface change from the volume report category to the selected surface category makes sense.
What is throwing me off is that you use the VolumeSummarySurfaceClassified values. I guess I would not intuitively think to uses these values. Why are these the correct ones to use and not the VolumeSummarySurfacetoSurface values, which is what you would expect if you are running the surface to surface function?
That is very close though. Thanks for helping with this.