There is a new feature in the 2015. It is based on ambient light sensor which I don't think you can override.
I love it for using the lights during day time rain storms. My state requires headlights be active when using wipers so I have to turn the lights on.
I set my dial to dim the gauges ~20%. When the lights are activated (either automatically or manually) the gauges will dim 20% ONLY IF the ambient sensor agrees that is it dark out. If the ambient sensor senses it is bright out, it will override your dial and keep the gauges at 100% brightness. I expect this is what you are experiencing. Try setting the dial to 50% and turn on the lights in the dark.
This is a nice feature when your state requires headlight/tail lights be on when there is a need for using your wipers, IE Rain/Snow.
In my experience the override only applies to the gauges. The head unit will dim to your dial setting even if the ambient sensor is overriding the dial for the gauges. I could be wrong, but I think I've witnessed this. It could be an issue if you set your dial very low. Gauges will be 100%, but your head unit will be very dim during daylight. If I'm correct, this was probably an oversight in design..
Previous Legacy's (ie my 2008), I would keep the dial at 20% and activating the headlights during the daylight hours left the gauges to dim and I had to manually dial the brightness up to 100%. Then I'd forget to dial it back down to 20% and next night drive I was adjusting it back.. I was constantly fiddling with it.