The volume of each channel depends on the gain as well as the channel master. There are lots of varying gain settings across the three channels. If you set the gains differently to what Randall Smith thought was 'about right', or to what some other user sets his to, or the guy in the shop, or me, then the channel masters on all our respective amps will need to be different, to even out our various gain settings to get them sounding the same volume. If, that is, we want them to be the same volume. (which we almost certainly don't)
Does that make sense?
It doesn't matter whether the masters are the same as each other- just twiddle them until it suits you.