I hear you, that's the way it's supposed to be which makes sense. And maybe all the dispatchers are all the same, but a lot of times they are much lower than the mobile units. I don't remember similar issues when it was just console or desktop mics, but I could be wrong.
You bring an interesting aspect to the discussion… how many people are listening/talking on a shared channel? Between two participants, there is no problem, they set the volume level for each other and you’re done. But add one additional person to the fray and you have the beginnings of a “round robin”, where all 3 or more people get in line, remember who they follow so they aren’t talking on top of each other, wait their turn and then transmit. When this happens, invariably, the volume knob gets excersized a lot if the volume level of others is not equalized. Digital communications (such as DMR) have the ability to set volume levels. The operator can take reports from others about how his volume level is too high or low and have the level for himself adjusted to meet the rest of the group on that channel. It should be relatively simple for supervised groups such as a police dept (I call this type of group a captive audience) where regulating how much the settings are adjusted and who makes any adjustments are identified. Of course this does not do anything for the volume equalization needs between officers when a new shift takes over the squad car… those must be handled the old fashioned way by telling the offending officer to speak up or back off the mic… something you hear quite often on the ham bands. The ham community using digital comms such as DMR, where any amount of reports of volume too high or too low are observed, may not get any satisfactory results because hams are not supervised anywhere near as closely as that “captive audience” police department. The operator or owner of the offending radio may ignore the reports and continue to upset others… in which case, he may be shunned by other hams (a tactic available to hams when they don’t adhere to common acceptance of accepted procedures. I’ve participated in some large round robins where it was necessary to keep one hand on the volume knob in order to adjust the volume of an incoming station's.