@@catherinewhisenand5678 was watching this today to calc a stockpile . You may this now, but leaving in case anyone watches later. From another video, if you choose stockpile it creates a plane based on the volume boundary (which can have changes in elevation), if you choose measured from elevation, it makes a plane set to only the elevation of the point you pick. So you can end up with more or less if you do not have a flat base to shoot from and choose the elevation option instead of stockpile.
