They're misrepresenting that data by saying here is "your" population number. It makes you think this number is uniquely assigned and no one else would be assigned that number. But they're assigning you a number based on world population on the date of your birth. And population doesn't count total births, it is the difference between total births and total deaths. I can demonstrate the absurdity of their numbers by looking at dates with relatively flat population totals over many years.
For the year Jan-1 1600, they state someone born on this day is the "537,000,219th person alive on Earth". For Jan-1 1700, they state someone born on this day is the "585,007,213th person alive on Earth". They're implying only 48 million people were born during those 100 years between those two dates. But total population in 1700 was 585 million? This means that the other 537 million people alive in 1700 were the same 537 million people alive in 1600.
And let's look at the last century. According to that site someone born Jan 1 1900 is the "1,578,042,704th person alive on Earth", and for Jan-1 2020 they're the "7,717,193,306th person alive on Earth". So all 7.7 billion people alive today get a number between 1.5 billion and and 7.7 billion. Looks like we have 6.2 billion numbers for 7.7 billion people. And with every person born before the year 1900 sharing the 1.5 billion numbers below 1,578,042,704, it quickly becomes apparent this method of numbering is incredibly inaccurate.
You can't get a number that corresponds to people that have ever lived by the current population totals. You would need to add in the total deaths that occurred before today as well, or the more obvious route, just use the figures for total births. That would give a correct count of "humans that ever lived".
This site has estimates of "How Many People Have Ever Lived":
https://www.prb.org/howmanypeoplehaveeverlivedonearth/If you were born after 1950, an accurate "Nth person alive on Earth" number would be at least 100 billion.
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The only reason lightning never strikes in the same place twice, is that the same place isn't there the second time.