In this case there is no answer, because the percentage is just made up by the LLM. There is no methodology. Your answer is inappropriately anthropomorphizing the LLM by imputing a methodology onto its "data collection" process.
How do I know the methods described in the source were correctly used here? I don't, but I also wouldn't know that based on whether or not a human wrote the webapp. I'd know that by checking the method applied on the backend matches that of the source. If it does, it doesn't matter if it was generated by using every 17th letter of the dictionary - it'd either align or not.
Preston, S. H., Heuveline, P., & Guillot, M. (2001). Demography: Measuring and modeling population processes. Blackwell Publishers.
In GP's case I believe they are correct the Hajnal source is not relevant to the Yangtze Basin but reading the first page should have made it clear the facts could be simultaneously true regardless where the percentages came from or if they were accurate.
Again, there are plenty of problems with the site. That it was clear these facts can't all be true simultaneously was just not one of them.
I believe we agree we haven't seen a valid citation for "the source in which 'x' came from in 'x%'" for this stat on the site. I hope the two of us also agree this is not the point I was disagreeing with.
Where I'm currently lost is why a demography textbook, which was a different citation in the same section, must be unable to explain dead newborns need not be counted in such a statistic on the basis the textbook was not also the source of the specific percentages displayed? That'd be like saying a statistics textbook can't explain how the median can be less than the mean on the basis it wasn't also the source for the numbers displayed.