Scripture often uses an economy of words to communicate a story. We may wish more words had been used to clearly spell out each detail. The point of the story is that Jehoash turned to God when he was in trouble but didn't have enough trust in God to ask for total victory. Elisha's response to Jehoash's actions indicate that the two men clearly knew what was being communicated.
In 2 Kings 13: 10-11 we see that Jehoash did not follow God.
10 In the thirty-seventh year of Joash king of Judah, Jehoash son of Jehoahaz became king of Israel in Samaria, and he reigned sixteen years. 11 He did evil in the eyes of the Lord and did not turn away from any of the sins of Jeroboam son of Nebat, which he had caused Israel to commit; he continued in them.
When things went really bad he turned to God's prophet for help.
2 Kings 13:14
Now Elisha had been suffering from the illness from which he died. Jehoash king of Israel went down to see him and wept over him. “My father! My father!” he cried. “The chariots and horsemen of Israel!”
In 2 Kings 13:17 Elisha clearly tells the king that the arrow fired out the window was the arrow of victory over the Arameans at Aphek.
So, one arrow equals one victory
In 2 Kings 13:18 Elisha tells Jehoash to strike the ground with arrows - the point being that the number of times he struck the ground would be the number of battles won. Three battles was not enough to win the war.
2 Kings 13:
19 The man of God was angry with him and said, “You should have struck the ground five or six times; then you would have defeated Aram and completely destroyed it. But now you will defeat it only three times.”
Jehoash communicated to Elisha, and God, where his his trust in God was - only three victories. As king Jehoash should have well known that three victories would not be enough but did not have the faith to ask God for what he needed.
Based on the story Elisha and Jehoash were clear on what was being communicated. Elisha was so clear on Jehoash's lack of faith that he was angry.