To be perfectly honest I have never seen an explanation for why they do this (am sure there is a logical reason, I just don't know the answer).
You certainly haven't been paying attention as I've explained it no less than two dozen times.
Yes, the scale with negative numbers is known as the 'relative' scale - as in everything is relative to 0. It has no meaning until you calibrate levels. If you follow the Dolby conventions and calibrate so that 0 is 'reference level' (105 dB peaks) then -10 dB on the display means that you are 10 dB below 'reference'; ie 95 dB peaks.
The 'absolute' scale (0 - whatever) is no different. It is jut not as convenient for determining at a glance how far away from 'reference' you are.
On the absolute scale, 0 would be silence. On the relative scale, 0 is reference level and -80 or -infinity( however low it goes) is silence.
Despite the fact that 0 dB may have some defined voltage level, like 'unity gain', it means absolutely nothing. The volume display could read A-Z and it would be the same (it just wouldn't make sense as we are conditioned to think of volume as a numeric quantity).