It's because of the difference between 'discard' and 'rescue'. A discarded card is only discarded by going to the discard pile. You can't discard half of a card, because if the card hasn't reached the discard pile, it hasn't been discarded.
However, rescuing is different. To rescue a LS in almost all cases is to place it in a Land of Redemption, with the one exception of this card, which specifically states that it must be rescued twice.
So Burial discards the whole card, whether or not a rescue has already been made on half of it. Son of God rescues half of the card, because it only rescues once on a card that must be rescued twice.
Make sense?
