Even though the punnett square calculates that a couple has a 50/50 chance of producing a male or female, there are slightly more females born in the human population. why might this be?

An egg is fertile when it ovulates. That's three days so male sperm gets to the egg quicker but it dies quicker too. So if couples have sex before ovulation by the time the egg is fertile the male sperm is dead which leaves the female sperm. This is why there may be more women than men.