It seems logical that your flash meter should give the same reading except there is the variable of what is reflecting the light back to the thyristor. If you have your flash meter propped up in a dark area then perhaps the thyristor is seeing a need for more light. If you have the meter set in a bright reflective background then the thyristor will see the need for less light. If you have your meter set in an area where the surface that bounces the light back to the thyristor is further away then the thyristor will see the need to pump more light in. Maybe you should try the test in a variety of settings.