Hi Nacho, it looks incredible but in fact is the way Revit uses to solve an irregular shape like a topographical surface.
In short, that cut/fill volumes are an "insignificant" proportion of the whole surface.
Think Revit has to draw "new lines" in your topo to solve the intersection between the current surface and the new pad. Before the pad construction was made, the topo surface was drawn by connecting nearest points with a known position/altitude. Once the pad construction is done, an interpolation method is needed to draw the topo on those points where the pad intersects its surface with a non-precise position/altitude.
The result of that is a little difference between the original and the resulting surfaces.
In addition to this, there is another issue involved to find a solution for the resulting topo surface by adapting its contours to the new conditions. I'm not an expert but I think it is similar to how we will solve an slope earth in real life, instead than a Revit bug. See the attached image where the modified topo has different shape than the original one (dashed red line). On your own, try to select de pad and drag its position to see how it affects the topo.
A further issue is pointed here because "the cut and fill volumes calculated by Revit are approximate" as said by @loboarch on a different post, just because an approximate value is needed when an irregular shape like a topographical surface is being calculated for the benefit of performance.
Regards.
