IfcOpenShell's OpenCascade kernel padded the box replacing a halfspace by tolerance x 1e6 (10 m at default 1e-5), so on a 1 mm ribbed sheet the boolean partly failed, a box corner survived, and z extent read 10.073 m instead of 0.073. Fixing the margin alone moved CGAL agreement only from 21 of 69 to 31 of 69 across an 80,177-element model.