Why is the volume so low when I connect Android Auto via USB?

[AC Question]

Media from my device plays at expected levels when I am using my Samsung Galaxy Note Edge and my Kenwood 9902S car audio system. But once I connect my USB cable and Android Auto enables, all the audio volume of all media music, phone etc is very, very low. I have to turn the audio system's unit up to maximum. This causes problems when I disconnect USB and the audio system reverts to Bluetooth audio or any other audio device, as the volume now jumps to its normal levels - which are deafening.