If you search for 'reference level' or 'relative volume' you'll find that this topic has been discussed many times and yet most are still confused. Perhaps rightly so because the numbers used for digital audio have different meanings in different contexts (particularly '0 dB').
1. 0 dB as a level means the sample value is at its maximum; eg. using 16 bit audio on a CD, 0 dB would be a sample value of +32767 or -32768. All values are expressed as a negative offset from that max.
2. 0 dB referring to Sound Pressure Level is the bottom of the scale and for all intents and purposes is silence.
3. 0 dB on the receiver volume scale is an arbitrary designation, but one that is very convenient as I'll explain once again.
There is no difference between a scale that goes from 0 to 100 ('absolute volume display') and one that goes from -80 to +20 ('relative volume display'). In fact, some receivers, like Onkyo, allow you to switch between the two at any time.
If one receiver uses the absolute display and is calibrated so that 60 is reference level and another receiver that uses the relative display is calibrated so that 0 is reference level then they produce the same SPL when the first one is at 60 and the second is at 0. Bump the absolute display to 61 and the SPL is reference+1; likewise bump the relative display to +1 and the SPL is reference+1. They were calibrated to the same reference level and being decibel accurate, incrementing or decrementing that number on the display changes the output by +/- 1 dB.
The relative display is just more convenient: If you calibrate so that 0 on the display is your reference level (whether you used Dolby Reference Level or some other level of your own choosing), then when the display reads -10 dB you can see at a glance that the SPL is 10 dB below your reference level. With the absolute display (and using 60 as the reference), when the display is on 50, you have to do the math and calculate 60 - 50 = 10, I'm 10 dB below the reference. That's it. The display could use aa - ZZ and the concept would be exactly the same.