B31.3 piping (refineries, petro-chemical plants) can have just as high pressure / temperatures as B31.1 piping.
My understanding that the reason for the lower allowables in B31.1 is to provide higher margin of safety / higher reliability to B31.1 piping, due to the increased consequences of failure.
If some B31.3 piping fails, typically the impact is to the immediate plant and to the Owner of that plant.
If some B31.1 (eg. power plant) piping fails, the impact will be the plant and the plant's Owner, but also may be to the general public and other industries that rely on the power from the power plant.
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Regards,
Dave Clark