I am not disputing anything that has been said about Harris systems, because I really don’t know much about them, but I have experienced the varying audio levels while monitoring Chester county on my scanners, but as a former LEO from a neighboring county, I do know that the OP’s observation about police leaving the mic in the holder is accurate. And it is not always out of laziness.
When newer radio systems were installed in the late 90’s, and we started carrying portables on our duty belt, along with lapel speaker mics on our shoulder, the lapel mic could create an undesirable feedback issue (we called it getting “pigged” because it created a squeal (yes, I get the irony)) when you used the mobile radio mic, as you brought it up to your mouth, putting it next to the lapel speaker mic. It was common to just key the mobile radio mic with it in the mic holder on the consolidator, so as to avoid getting close to the lapel mic, so as not to pig yourself, which would be met with ridicule from your brothers and sisters in blue on the side channel.
Of course, it was also common during the days of carrying Nextels, that when you would get a dispatch and respond, your fellow officers would “hit you up” on the Nextel and shout some inappropriate comment while you were trying to reply to county. Fun times