Why is it (sometimes at least) cheaper to book a return flight using Avios rather than the same flights as two one-ways? Just made an open-jaw booking by phone (EDI-LHR-DFW, ATL-LHR-EDI) in CW and paid about £70 less than if I'd booked the same flights online as two one-ways (and I even paid the £15 phone booking fee). Shouldn't taxes and charges be same either way?