How the difference is computed
This page describes what Compute difference actually computes: the method, the settings it uses, and how the numbers in the panel follow from its output. It is here so you can judge whether a result is trustworthy, and explain it to someone who asks.
The method in one sentence
Both revisions are rasterised into 2.5D elevation grids on an identical pixel grid, revision A’s grid is subtracted from revision B’s, and the result is masked to your boundary and stored as a GeoTIFF that everything else reads.
2.5D, not 3D
A 2.5D grid holds one height per cell - the smoothed height of the survey points that fell in it. It is the right model for a seabed and the wrong model for anything vertical. Overhangs, the sides of a pipe, the walls of a structure, the underside of a mattress: none of these are surfaces the grid can represent, so change on them is not measured. The panel repeats this caveat in Method & calculation settings for exactly this reason.