This is expected, as the Radiation efficiency will depend on which modes you excite. 
From help:
"For an FE subsystem the radiation efficiency indicates the in-situ radiation efficiency (rather than the radiation efficiency of the resonant modes of the panel). The radiation efficiency of a structural FE subsystem depends on its response (i.e. depends on the loading, damping, coupled subsystems, etc...)."
            
                                
                    
                    Ricardo
                    Alvarez
                
                                
                    
                    Vibro-Acoustics