The volume display is just a scale. Regardless of whether it uses the absolute scale or relative scale, it is the same. If the volume control is decibel accurate, as most newer receivers are, it does not matter which scale you use.
A change from 20 to 30 on the absolute scale is the same as a change from -30 to -20 on the relative scale (10 dB change). If you download the manual for Onkyo receivers that allow use of either scale, it will say that 82 on the absolute scale is the same as 0 on the relative scale and that should help to make it clear.
I've tried to explain the idea of scale many times in response to questions on reference level or absolute vs relative volume display but it seems that many think the number on the dial has some special meaning - it does not.
You could label the volume display from A to Z and it would still be the same, it just wouldn't make any sense.
