Mesh.RecalculateBounds

The documentation for the function contains this comment... "Assigning triangles will automatically Recalculate the bounding volume."

Experimentally I am seeing differences if I manually call this after assigning triangles?

 mesh.triangles = newTriangles;
 Bounds b1 = mesh.bounds;
 mesh.RecalculateBounds();
 Bounds b2 = mesh.bounds;
 if (b1 != b2)
 {
     Debug.Log("Bounds "+b1+" => "+b2);
 }

Bounds Center: (19.5, 0.8, 19.5), Extents: (28.1, 0.8, 28.2) => Center: (19.5, 0.8, 29.4), Extents: (28.1, 0.8, 38.0)

Is the documentation incorrect, or am I mis-understanding something?

Paul