What women have less than men?

We are in the XXI century but the developing isn’t everywhere and, because of that, the countries of the world are divided: where are more difficulties and poverty, they are called Developing Countries and the countries financially stable where people have a good life, like a house, employment and resources as water and sewage they are called Developed Countries.
If I was shocked when in the News I saw something like “Children in India are sold by the parents to go work 16 hours in a factory” and this happened in a developing country, I’m shocked too when I saw in the News the wage differences between men and women in Developed Countries.
For me and, of course, to more people, this is just discrimination. It’s antiquated because if in the human rights says that men and women have the same rights, and if a man and a woman who works in the same place, doing the same with the same working hours, why should that man earn more than the woman? It’s discrimination sexiest without any explanation. The wage difference in UE in the year 2007, on average, between men and women is 17,4%.
Many people said that women have fewer possibilities to advance because they are woman. Now, can somebody explain me why these differences happen? I really want to know.