Confused about stereo/mono

I recorded a video. On one channel I had an external mic and on another channel I had an internal mic. The internal mic was used as a back up. I only want to use the external mic. I bring the video to final cut x and it says it's a stereo recording. All good. I can see the levels different for each channel. So I want to "turn off" channel 2 as that's the internal mic. I change stereo to dual mono. Now I see both channels and and I can turn one off BUT. Before I turn one off, the levels are now the same for both channels, why is that? Am I hearing a combination of left and right on both channels? If so why am I not hearing any difference, when I turn one of them off? I would expect a massive difference but I hear none. Why is that?