If the LORD said, "I will add fifteen years to your life" (38:5), why did Isaiah tell them to lay a cake of figs on the boil?

The chapter tells both and doesn't say how they go together. The LORD says, "Behold, I will add fifteen years to your life" (38:5), and the heading of Hezekiah's writing says he "had recovered of his sickness" (38:9). At its end the chapter tells what Isaiah had said: "Let them take a cake of figs, and lay it for a poultice on the boil, and he shall recover" (38:21). Isaiah's telling doesn't have the words "Behold, I will heal you" (2 Kings 20:5), and it gives the figs no reason. So the chapter gives the added years to the LORD, tells of the figs laid on the boil, and says no more about what the figs did. The chapter on 2 Kings 20 asks the same question of its own telling.