The difficulty of reconciling this verse with 1 Samuel 17:55-58, is met thus: The words here are the ultimate sequence of David’s first visit to Saul, and of his skill in music, and are therefore placed here; but they did not really come to pass until after David’s victory over Goliath (see 1 Samuel 18:2). It is quite conceivable that if David had only played once or twice to Saul, and then returned to his father’s house for some months, Saul might not recognize him.
Related Commentaries of 1 Samuel 16:21
1 Samuel 16:21
21 And David came to Saul, and stood before him: and he loved him greatly; and he became his armourbearer.