I drove this make of car for a while; there's an optional head up display where the up and down buttons here let you cycle through contacts/the song queue/radio stations. I'd imagine it's the same interface without it, just displayed somewhere in the car where you're not looking while driving.
Having it so that up/down moves you up/down through the list when there's a visual display is way more intuitive than up/down being volume - frankly the volume bar on Windows, Mac, many TVs etc. goes from left (quiet) to right (loud) anyway