It is all due to seniority. I have a couple of friends who fly for Qantas. You will notice that the older crew are on the NRT and LAX runs, generally because their allowances that they receive are higher on those routes, therefore they bring home more money. The more senior they are, the more flights they get that they bid for, to the destinations they want. You will get the younger crew on the less attractive destinations like JNB, MNL etc.
Now they have mixed the First and Business Class crews, you may notice some difference.