I do not know how BA does it but with SQ it depends a lot on the type of passenger and his membership and also the connections. The connections play a major role. If you are connecting they try to keep you. If you are Elite and it was Solitaire then you fly no matter what. Gold they will not bump off. So it is the poor guy going on holiday with the lowest fare and no Frequent flyer is the first choice to get bumped. He is the one who may be mostly accepting the compensation and the opportunity to fly in a better class,