The root of this is that having a daughter was less culturally valued, people chose sex-selective abortions (http://en.wikipedia.org/wiki/Sex-selective_abortion#China), which led to a skewed gender ratio for the population. Many rural men struggle to find wives because of the shortage. If women/daughters were more valued, there would be more women, and poor men would have an easier time finding a wife. So even though there are more men on the bottom, it was caused by valuing men over women.