I was wondering the same. I guess it comes from this "UN figure":

> The UN figures include a mixture of city proper, metropolitan area, and urban area.

https://en.wikipedia.org/wiki/List_of_largest_cities

I haven't looked into the details of that definition.

But there is a somewhat standard definition to "metropolitan area" derived from something like "area where there is at least X per square km"

So it's not related a somewhat random definition of a "city" and its borders.