I think it's because Sal didn't know the whole story (how David died and the events after). Dave reveals this fact to Sal when Sal shared his sister's story but only the fact that David's dead. Everything else was omited and therefore Sal is still suprised.
EDIT: Probably Sal is suprised because it's the only problem that bothers Dave all along. He is still shaken up by it and didn't get over it. And Dave didn't share it with him and the group.