When the testes produce sperm, they produce an equal number of sperm carrying the X chromosome and the Y chromosome. However after this a couple of factors come into play. One factor is that Y-carrying sperm weigh a little less than X since the X is a much larger chromosome. This give a slight advantage to a Y-sperm fertilizing the egg, so you'd expect a higher percentage of XY fertilized eggs. However, these eggs are also more vulnerable to any genetic errors on the X chromosome since there's only one copy. So a lot of XY embryos spontaneously miscarry early on in the pregnancy. The end result is the approximately 52%M/48%F live birth rate. By puberty the balance between men and women is close to 50/50 since men have higher death rates in childhood.