Not sure. Am presuming because I keep it in a leather slip case, Siri thinks the phone is against my ear, so is listening for a voice instruction.
I thought that option being on just forced it for when turning on siri you could just begin talking otherwise you had to hit the microphone button. I didn't think it was always searching for your ear but I could be wrong. If it is I wonder if that is part of my drain issue. It is dying much faster than I expected.