You have to think about the math in terms of generations. It takes a couple of generations for the processes that reduce child mortality rates and increase adul

You have to think about the math in terms of generations. It takes a couple of generations for the processes that reduce child mortality rates and increase adult lifespans to take hold. So there's a boom generation that will be much larger and live much longer than originally expected. As the life expectancy changes from 50 to 70, that's a lot more people - people born a half-century earlier.