I'm guessing here, but we know that the mic system on the wing is inductive. If you are sitting quietly with the intercom on, tap the top shelter with your finger and you'll hear that the noise will be picked up by the intercom system (you'll hear the tapping in your helmet!). So, when the horn blows it should also be picked up by the audio system. I'm thinking that at squelch levels over 11, the noise picked up by the mic is not enough to break squelch but at lower levels the noise is enough that it is passed through like a strong signal. Maybe?? :roll:
Surely someone with a much better understanding of how all this stuff works will point us in the right direction!