The mythical million comes from estimates built upon estimates that have spread over the years like Chinese whispers through the NGO community and, later, the internet. Paul Currion laid out how this works two years ago, in his essay "Lies, damned lies and you know the rest":
In the absence of actual data (such as an official census), NGO staff make a back-of-envelope estimate in order to plan their projects; a postgraduate visiting the NGO staff tweaks that estimate for his thesis research; a journalist interviews the researcher and includes the estimate in a newspaper article; a UN officer reads the article and copies the estimate into her report; a television station picks up the report and the estimate becomes the headline; NGO staff see the television report and update their original estimate accordingly. All statistical hell breaks loose, and the population of Kibera leaps ever higher.
Every actor at every stage has a motive for using the upper end of that initial estimate, rather than more conservative figures – planning, funding, visibility, and so on – but no single person is responsible for inflating the figure progressively further from reality.