Inaccurate site model cut and fill volumes

[Viridian]

I'm creating a site modifier on a clean model showing c&F as nil before I start

The Modifier is set at a level 118000(mm) to form the base of a hard landscape area where existing levels vary between 118160 and 118020 - therefor it is all cut, yet the C&F data shows:

Cut 19.4 m3     Fill 36.8 m3    with a net fill of 17.5m3 which is nonsense.

 

The Modifier is set to Pad, Apply to Existing - though the data is the same if I change to Proposed

 

When I update the model the readings are different  C=21.2m3, F= 55.2 net fill=34m3

 

When I delete the Modifier the site model return to 0.00 C&F

 

any ideas?