If you want to get technical, this isn't a continuous surface but weave of strings. And something that makes this more complex is the location of the string intersections has a static/dynamic friction component that is difficult to model analytically. But I think we accept your approximation if you are looking for numeric results.
One can make a similar heuristic argument, though, as to why the surface you observed should follow the minimal-surface equation.
EDIT, as you've updated your comment:
> Isn't Poisson's equation basically describing a minimal surface for small z?
Small changes in z, I think.