I wouldn't think evolution has anything to do with it. Quite simply, the face with lower contrast is perceived to be flatter. (The reason is purely geometric -- a curvier surface contains a wider range of angles-of-incidence with respect to the lighting source, thus resulting in a higher-contrast image.)

By placing the two images next to each other, our brain assumes that they are illuminated by the same lighting source, immediately tags one as "round" and one as "flat". Women, by the very nature of having "higher cheekbones" (and typically more facial fat), have rounder cheeks. Thus, if the brain tries to tag the two images as different sexes, it's much more likely to tag the higher-contrast (rounder-looking) face as female and the lower-contrast (flatter-looking) face as male.

Helping (hurting?) the situation is also the fact that the black level of the eyebrows was not kept constant when adjusting the contrast, thus causing them to lighten and look thinner in the higher-contrast image. Women tend to have thinner eyebrows (at least in modern society, due to the fashion trend of eyebrow plucking), no doubt lending weight to the brain's decision.

As to why women have higher cheekbones and more facial fat, well that question I'll have to defer to an evolutionary biologist :)