The volume setting on an amp is absolutely no indication of the the output being used, it is really an indicator of the input level.
Amplifiers have fixed maximumum outputs, a high level input signal, from some CD players for example, needs much less 'gain' to reach maximum output than a lower level signal such as a tuner or phono stage. Hence a high output CD player might sound very loud at 10 o'clock but drive the amplifier into clipping at 12 o'clock whereas a lower output source component might need much higher settings on the same amplifier volume control to reach the same output levels.
It is also the case that many amplifier manufacturers deliberately make their volume controls in such a way that the amplifier gets very loud, very quickly. This may impress the uninformed but is just a con, the amplifier can only produce its maximum output irrespective of the setting of the volume control.
In addition a a more gentle, wide ranging control makes accurate volume setting much easier and eliminates the issue of poor channel balance at low levels that plagues conventional ' ganged' analogue volume pots.