Gosh yes, it's almost like different places could have different amounts of use of Ebay, I can't imagine a world where that would happen :(
It is an interesting topic, but I do think the numbers will be off - maybe by quite a lot -
but I suspect it may be an underestimation.
Different countries collect their numbers in different ways. For some countries, it will be very very accurate - they register all births, register all deaths, they count up the number of people in tiny areas, add them to get small areas, add them to get medium areas, big areas, one division of the country, a region of the country, then the country (usually). It is checked and checked again.
For some countries this would not be a good use of resources and they would be better putting the money and energy into other things (like healthcare, clean running water, food, shelter).
Some places don't register.
It is a guess at best but that is all it can be.
There are figures as to when the population will start to decline but the year keeps changing.
It is not that far away.