I don't understand the "you'll be the Xth billionth person the be alive in the world" at all. How does this number go up over time? When you're born, how can yo

I don't understand the "you'll be the Xth billionth person the be alive in the world" at all. How does this number go up over time? When you're born, how can you be the "first" person to be alive? How am I heading towards eventually being the 9 billionth person alive? I guess it means "oldest person?" If so, why not just say that? Clicking on the snake chart thing also makes the age histograms on the left move to arbitrary places. I click on the dot around 43, and the age chart shows 41. I then click on the dot around 50 and the age chart shows 8.