Guys; as I said before if you look at FCOM 1.24.10 page 2, the supply from the APU joins "downstream" of the gen line contactor for Gen 1 and as such is not an independent source of electrical power as a fault could exist between there and AC Bus 1, hence Cat 3 Single.
The A330 however, goes straight into AC1 without sharing any gen 1 circuit - and independent source of electrical power and therefore Cat 3 Dual when running with an engine generator!