Most likely there's a DC-to-DC stage internally that provides a clean and filtered 12V from whatever the 15V DC input is. This gives them a bit of headroom for

Most likely there's a DC-to-DC stage internally that provides a clean and filtered 12V from whatever the 15V DC input is. This gives them a bit of headroom for power design and ensures that regardless of the DC input, the device is getting clean DC power.