I met a journalist from Karachi once. Did some traveling together. They were telling me how the population of Karachi is widely disputed, how people throw different numbers around all the time and the real number is hard to determine. Judging by this article, that's a common problem for a lot of information in Karachi. I'd guess that's due to political instability in the region preventing accurate information gathering and distribution. I find that to be the more interesting part of this story.