Why is the population of female is lower than that of population of boys?

My Biology teacher told us that it is actually the Y-chromosome in the male sperm cell. Roughly half of the sperm cells are equipped with it, the other half carries a X-chromosome. Those determine if the baby becomes a boy (Y) or a girl (X). The y-chromosome is actually just a X-chromosome with a missing strand, therefore it's lighter and gives the sperm cells equipped with it an advantage in speed. The first cell to hit the egg wins, so y-chromosome carrying sperm cells have a slight advantage. It's marginal but it's there.

Hope I could help.