Global Mapper v25.0

Calculate volume in Gallons results are questionable

geomeskin Global Mapper User
I just used the "Calculate Cut/Fill Volume" tool to calculate the volume of a water pit.  The pit was designed to hold 1,000,000 gallons.
I'm calculating the volume using a LiDAR data set and the contours generated from that data set. 
In the resulting table:

  • I get acreage values that are spot on.
  • elevation contours that are spot on.
BUT...the Gallons (and I tried Barrels too) are off by almost half.  I can't figure out why.

I've checked the projection of the data and the units.  Everything is where it should be.
Using the acreage values I get the correct number of cubic feet and then gallons.  But those calculated values don't at all match the gallons calculated by GM.

My question is why the GALLONS/Barrels calculations provided by GM are so off when everything else seems to be right?