The shell in Benchmark LE4 is the result of revolving a hyperbola. Since the normal offset from an hyperbola is not another hyperbola (or any other simple curve supported by STEP geometry - ISO 10303-42), the thick shell cannot be exactly represented in AP203.
One approach would be to take a parallel offset, but this results in a thickness variation of about 30%. Instead, 2 hyperbolae have been created - one to generate the inner surface and one to generate the outer surface. The hyperbolae have been chosen to give the correct thickness at the middle and at the ends of the curve.