Relative position in poker is your position in relation to the pre-flop raiser. So having SB in there can definitely occur a good amount of the time. You’re assuming the pre-flop raiser is going to c-bet, so your position in relation to closing the action post flop is what relative position entails.
Ex: MP open raises, BTN calls, and you call in the SB, BB folds. You have relative position to the pre-flop raiser because BTN will act before you post flop.