Why are more males born than females?

Worldwide, there are 107 boy babies born for every 100 girl babies. This skewed ratio is partly due to sex-selective abortion and "gendercide," the killing of female infants, in countries such as China and India where males are more desired. But even discounting those factors, the completely natural male-to-female sex ratio still hovers around 105:100, meaning that women are inherently more likely to give birth to boys. Why?

Several factors influence whether a sperm containing a Y sex chromosome or one containing an X chromosome will be first to fertilize an egg, including parental ages, their environmental exposure, stress, the stage in the mother's ovulation cycle and even whether she has had children previously; all these forces combine to set the average sex ratio at fertilization at 105:100.