Hello; One of my topics of interest from the 1970's has been human population. Probably dates further back to the late 1960's during my quest for an undergrad degree in biology. I still have at least one of the ecology textbooks of the time. I know it was sections concerning population dynamics which sparked the interest.
Anyway the gist of the link seems to be that rural populations have been seriously undercounted. That there may be perhaps a billion or more of an undercount in commonly used data bases.
At least one source referenced is the numbers of displaced people when dams are built. Apparently, there are more registered than thought when the dam projects start.
Of course the notion is challenged.
