Because *A inventory is released at a low price, it is scarce (being largely used to provide awards at a heavy discount on lightly loaded flights or very far in advance). In other programmes, people snap up rewards up to a year in advance because of this (I've done this with QFF in F SYD-LHR).

NZ inventory is fully open with APD used as a currency for any flights, so because it has universal value across NZ, it costs more to use. NZ inventory scarcity is managed by the price of fares.