They ask for backpay not really expecting for backpay, just to give them wiggle room to settle in court for better rights from that moment on.
Last and only time I had to sue someone (and won) my lawyer told me what the usual result of cases like mine is, then we asked for that and like, 20% extra. Then on the mediation we "negociated" for the amount we were really expecting to get.
(This is all personal speculation, Im not a mod, clearing that up just in case).