It may not be. Warn/simmer or seat leakage is sometimes mistaken for set pressure. Visible or audible leakage or system pressure drop is not set pressure. The correct definition of set pressure is: For liquid service, first vertical steady stream. For liquid service, first vertical steady stream.
For some valves in air/gas service (Models 230, 330, 330S, 333S, C776), First audible. Variance of set pressure is allowed, i.e., a Section VIII air valve with a nameplate of 100 psig set pressure may open from 97 psig to 103 psig but will be factory set around 102 psig.