Here is one answer:
The vanishing twin syndrome.
Vanishing twins occur in up to one out of every eight multifetus pregnancies and may not even be known in most cases. “High resorption rates, which cannot be explained on the basis of the expected abortion rate…suggest intense fetal competition for space, nutrition, or other factors during early gestation, with frequent loss or resorption of the other twin(s).”
Celebrities tend to have good medical services. The better the services, the better the chance for both twins to survive as there is no need to fight for nutrition and everything else is better monitored. If a mother-to-be is not getting the proper nutrition and medical supervision, the chance is much higher that one of twins will not have enough to survive.
The truth is if it wasn’t for vanishing twins, the amount of twins born would be at a higher rate than that of celebrities. Which goes to show just how important it is to have proper medical supervision as well as the right diet to ensure every fetus survives.