Albert Barnes Commentary 1 Samuel 17:15

"Now David went to and fro from Saul to feed his father`s sheep at Beth-lehem." — 1 Samuel 17:15 (ASV)

David went ... - “Was gone,” referring to 1 Samuel 16:19–20. Had he been Saul’s armor-bearer at this time, it is highly improbable that he would have left him to feed sheep.