This image is a fairly classic, even highly compressible (537 kB), fairly efficient, RGB to Grayscale implementation graphical representation. Some graphics programs will convert this image into a perfect black to white gradient. Superior sRGB-based(or similarly gamma corrected CS) programs should not(Photoshop, for instance).
Irfanview uses the same method. Test it out for yourself.
Formula: Gray = (77 * Red + 150 * Green + 28 * Blue) / 255