K-3 Flash Master and Controller confusion

Hi folks, hoping someone can clarify something for me. I just acquired a Metz 48 AF-1 to use with my K-3. Both devices have the latest firmware. I was playing with the wireless triggering function, switching between Master and Controller on the camera to see the difference. Now, everything I have read indicates the Controller option will not contribute to the exposure, whereas Master will. So I have taken shots in both modes, with the camera pointing towards a mirror. In both options, the built-in flash shows in the mirror, but strangely in the Controller shots, the built-in flash is significantly brighter than in the Master shots. I would have expected this to be the other way around if, in fact, the Controller option isn't supposed to contribute to the exposure.

Can anyone illuminate me please?

Thanks in advance for any help.