hope that makes sense - tried to use really simple numbers - it's far more complex, but essentially ANY CCB top up you're entitled to at tax time will be cut in half as you get 50c in the dollar CCR - so if you get $100 extra CCB at tax time, you will have been over paid CCR $50, and would get the difference ($50) paid to you
it's done this way rather than paying you the full CCB top up then raising a debt for CCR for half the amount.