Seems to me like most are in the south, close to the coast
https://en.wikipedia.org/wiki/Nigeria#DemographicsHowever it makes sense for most of the demographics of a country to live in the North, if the geography and other factors benefit it, like in countries that have coasts on the North side, like Egypt
https://en.wikipedia.org/wiki/Egypt#Demographics. It isn't a great mystery, people live usually close to the water as it helps with transportation.