My speculation would be that the program you are comparing against is plotting results. In cases where you expect a change in z values that is relatively small compared to the x and y range, you might want to exaggerate the z (scale it up) in order to be more easily understood. Like
surf(X, Y, exaggeration_factor * Z)
The dz I would speculate to be the interval at which it samples the Z for plotting (or calculation) purposes.