CALC_VOLUME_BETWEEN_SURFACES producing different results than CALC_VOLUMES of subtracted elevations

I wish to measure the volume difference between two elevation models of the same area. I can either a) measure the volume between the two surfaces or b) subtract the surfaces from each other to create an elevation model of the differences, and then measure the volume change from z=0 plane.

Unfortunately when using Global Mapper scripting the results are wildly different. The two scripting files for each method are:

a)
```
GLOBAL_MAPPER_SCRIPT VERSION=1.00 SHOW_WARNINGS=YES LOG_TO_COMMAND_PROMPT=YES
LOG_MESSAGE Starting...

IMPORT FILENAME=%DEM% TYPE="GEOTIFF"

DEFINE_VAR NAME="MINE_SHAPE" VALUE=%SHAPEFILE%

IMPORT FILENAME="%MINE_SHAPE%"
CALC_VOLUMES FILENAME=%MINE_SHAPE% OUTPUT_FILENAME="%CSV%"
```

```
GLOBAL_MAPPER_SCRIPT VERSION=1.00 SHOW_WARNINGS=YES LOG_TO_COMMAND_PROMPT=YES
LOG_MESSAGE Starting...

IMPORT FILENAME=%DEM1% TYPE="GEOTIFF"
IMPORT FILENAME=%DEM2% TYPE="GEOTIFF"
IMPORT FILENAME=%SHAPEFILE% "

CALC_VOLUME_BETWEEN_SURFACES LAYER1_FILENAME=%DEM1% \
LAYER2_FILENAME=%DEM2% \
AREA_FILENAME=%SHAPEFILE% \
OUTPUT_FILENAME="%CSV%" \
```

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