Can you please share the links to the maps (or at least the names) and screenshots fo the stockpiles you are talking about. We have no way of even guessing which ones you are talking about here.
Please include screenshots of the polygons you are using to make your measurements as shown here:
Note that LOTS of points are used in the definition of the measurement polygon. If you are using just a few points that is the best way to get inconsistent results.