Particularly in musical terminology, there must be dynamics at some areas. Like for instance you may see on your musical scores like p,f,mp,mf,ff,pp. You don't play straight volume all the way. That shows how the event of the song being played upon. So for your case, if you know where's the climatic changes should be it is alright. I think James is trying to say that some people aren't consistent as they are unsure or not confident enough with his or her singing.
The monitor speakers as it's called are for the musicians on stage to monitor how they sound. The settings are different from the main speakers out for the crowd. A good sound engineer, would know how to balance the output speakers to be sounded optimum. But sometimes a good sound engineer can't help much on the sound of the band too. Example if the band is blasting on their amplifiers, in turn he will lower down the volume at the mixer. Hence, the mixing quality plus the raw amp output became unstablely unbalance. So, it goes both way. That's why the soundcheck is very important. Musicians will set their volume ideally on their amplifiers. At the same time, the sound engineer will see what setting suits the band best. So when time to play, you just stick to the volume. On the mixing side, if the some instrument volume got drown due to some dynamics on other instruments the sound engineer will push up the volume accordingly.
Saluti!
Tetrgrammaton