Why is the alpha shape calculating the same volume and the convex hull volume?

Question

I am using the CGAL library of the C++ language to calculate the volume of the alpha shape. I have completed it, but why is its volume the same as the volume calculated by the convex hull.

this convex hull's code

Polyhedron p;
CGAL::convex_hull_3(points.begin(), points.end(), p);
double volume =  CGAL::Polygon_mesh_processing::volume(p);

this alpha shape's code

double vol1 = 0.0;
    std::list<Cell_handle> cells1;
    as.get_alpha_shape_cells(std::back_inserter(cells1),
                             Fixed_alpha_shape_3::EXTERIOR);
    std::list<Cell_handle>::iterator it_cell1;
    for(it_cell1 = cells1.begin(); it_cell1 != cells1.end(); it_cell1++) {
      Cell_handle cell = *it_cell1;
      Tetrahedron t(
          cell->vertex(0)->point(),
          cell->vertex(1)->point(),
          cell->vertex(2)->point(),
          cell->vertex(3)->point()
      );
      vol1 += fabs(t.volume());
    }

If the program is correct, the volume of the alpha shape will be smaller than that of the convex hull