They ask for the worksheet to check that you're not cheating. They don't check the pixel values. I think much of the charm of the image is seeing the errors.

By the way, there's more going on in the shader algorithm than you might expect. Here's an explanation of the worksheet:

  u, v are coordinates relative to the center of the image
  h is radius from center, squared
  Section B generates the ball:
  B3-8 generates the reflected color on the ball.
  B9-11 applies the diffuse illumination to the ball.
  B12 adds the illumination highlight.
  Section C creates the ground:
  C5 puts a shadow directly under the ball.
  C13 is the cast shadow of the ball.
  Section D creates the sky with a simple gradient
  Section E converts the image from two-color to three-color