What is the twist in the story "The Last Leaf"?

The twist, or surprise, in O. Henry's story "The Last Leaf" comes at the very end. Johnsy has been expecting to die when the last ivy leaf fall off the vine attached to the brick wall of a neighboring building. It seems likely that she actually will die when this happens because she believes in it so firmly. Johnsy is described as small and frail. 

A mite of a little woman with blood thinned by California zephyrs was hardly fair game for the red-fisted, short-breathed old duffer.

If Mr. Pneumonia can kill much stronger people by the hundreds, why should he be expected to spare little Johnsy? The reader is practically convinced that the last leaf will fall any moment and that Johnsy will really pass away when that happens. 
But somehow the brave leaf clings tenaciously to its place on the denuded vine. If we believe that Johnsy can will herself to die when the leaf falls, then we can believe that she will decide to recover, and will recover, when the leaf doesn't fall. The actual "twist" in the story does not come when the leaf doesn't fall, but when Sue reveals that it couldn't fall because it had been painted on the wall in the middle of the night.

“Mr. Behrman died of pneumonia to-day in the hospital. He was ill only two days. The janitor found him on the morning of the first day in his room downstairs helpless with pain. His shoes and clothing were wet through and icy cold....Didn't you wonder why it never fluttered or moved when the wind blew? Ah, darling, it's Behrman's masterpiece—he painted it there the night that the last leaf fell.”

It should be noted that O. Henry takes considerable pains to keep the reader from suspecting that Old Mr. Behrman might get the idea of painting a fake ivy leaf on the brick wall in order to keep Johnsy alive and inspire her to recover. The author makes Behrman an old man. He is a heavy drinker. He has given up painting and only models for other artists. And he expresses extreme skepticism about the idea of anybody dying when a leaf falls off a vine.

“Vass!” he cried. “Is dere people in de world mit der foolishness to die because leafs dey drop off from a confounded vine? I haf not heard of such a thing."