Now it fell on a day, that Jonathan the son of Saul said to the young man who bore his armor, Come, and let us go over to the Philistines' garrison, that is on yonder side. But he didn't tell his father.
Exactly the same thing is true here.  The previous chapter revealed that God had rejected Saul's continuing dynasty; and whatever Saul did afterward was of little or no importance whatever, except that in a brief record of his mistakes, the instruction of future generations might be accomplished.