Simple. ASME II Part D gives the Max allowable stress for ONLY the bolt material, whereas ASME PCC-1 table gives the ASSEMBlY Bolt stress that takes into account the allowable stresses for flange (without bending) and gasket (without exceeding the max compressive stress).
In other words, when you make a joint with different materials with different stress values, and where you send the muscle man to tighten the joint, you must limit the load on the bolts to protect the other components.
75 ksi is for the example with B7 bolts, which is about 70% of min specified yield. The actual selection of the 70% maximum is arbitrary, because 75%, 80% or 85% seemed too high for general application. If you refer to section 10, it states that this range can be exceeded for specific cases when assessed by a qualified engineer.