Because of the different product forms and processing. If you review ASME Section II Part D you will see the allowable stresses can vary by specific product form.
Thank you, Allowable stress of SA351CF3M > SA182F316L,as per ASME SectionII-D Casting stronger than Forging, but in ASME B31.1 Forging stronger than Casting, Why
Different codes, different rules. Code committees take decisions for which the rationale or explanation isnt necessarily always available to the public years later. Maybe someone here knows, maybe not. Thats how codes work.