he doesnt absorb either of them. In black/white 1 kyurem was a pokemon and the two brothers argued over truth and ideal and kyurem split in half creating zeekrom and reshiram meaning that kyurem can only exist by combining both zeekrom and reshiram.
I have two guesses as to why kyurem looks different on the covers of the games. one guess is that in black/white 2 they combine again and later for whatever reason only one part splits off leaving kyurem to resemble one or the other.
the other guess is that zeekrom and reshiram combine forming kyurem and for some reason one of them dominates while keeping whole so kyurem resembles one or the other