my teacher is a hsc english judger (ie after all the initial marking is done they sit in a group n alter the bands n what students did to achieve bands and wateva) and although he didnt specifically say each text gets scalled differently, the way it works is they sit there and look at what students getting eg 18's, 19's, 20's did, then compared them to the 15's 16's and 17's and then they'll move the bands to get a more even outlay...
so what i'm saying is although you might get a 15 fr your essay initially, they may bump you up or down depending on what you did. I dont understand how they can do that for a whole module...so i'm guessing its done for each text (or question)