Why do valves with the same pressure class get tested at slightly different pressures because the body material is different? (API 598)

F316 and A105N for example.

Looking at table 3, for check valves, other test pressures, note that the pressure references b below the table that includes 110% of the maximum allowable pressure at 100 degree F for steel, alloys and stainless.  Type 316 stainless steel is rated for less pressure in each Class rating that A105.

Different metals have different strengths.  Castings are the same thickness.  Therefore a casting made of A105 for example would be able to hold a higher pressure than one made of 316.  Most people are unaware of that because the majority of valve manufacturers only make castings out of a few metals.  The manufacturers test to API 598 using the pressures for the lowest strength metal that they produce.

fangl40 is right, the ASME B16.34 specifies Pressure-Temperature ratings. The test pressure for shell test is 1.5 times of max allowable working pressure under room temp which is -29 to 38C. Different material group has slightly different working pressure even under different temperature.

Test pressures are based off the 100