The volume steps reflect a rough scale of human perception of sound levels.

The comment about the seemingly random fluctuation is fair, though. Supposing (just supposing) the human perceptual response DID look like that. The exact positions of the little peaks as heard by the listener would depend on the gain of the amplifier. If I chose to have my amp gain a bit different, then the peaks of the empeg would line up with the troughs in my ear/brain combination. This would equate to me pressing volume up a lot of times for no perceived increase in loudness, and then all of a sudden I'd press it once and the music would seem much louder to me.

Now, I don't believe the ear's response does have all those sharp peaks, but I'm happy to be proven wrong. But either way, because of the unknown amplifier / listening environment gain, it really only makes sense to have a very smooth curve, I reckon.

Richard.