Why can I not control the volume of my ASUS VE278 monitor with my keyboard buttons?

I have an ASUS VE278 hooked up to my Mid-2010 Intel i7 MBP with an HDMI cable and displayport adapter. I also have a bluetooth magic mouse and wired USB full size Apple keyboard attached. When I press any of the volume keys on the keyboard I get the standard volume pop-up but the volume bar is maxed out with a cirle slash underneath. I'm running 10.8.2.