Nigeria is supposed to be among the top gambling country globally because I saw an article early this year that states that the the Nigeria online gambling market is estimate at €675.1m in grass gambling revenue this year. Secondly, I learned that Nigeria ranked the third country in Africa gambling market, while South Africa and Kenya rank 1 and 2, and from the OP's data, those country are not even added. Some of those data online are not completely accurate, that's what I observe.
If we are to judge base on the rise in gambling in the country called Nigeria we will all argue that we are suppose to come out among the top 10, but most of the Africa countries not just Nigeria is not among the top on that list, most of this data analysis make use of different data which they get from different places which is why we don’t have to believe everything that we get from one source, base on the data they gather their conclusion might be correct if others who are also more focused on Africa countries present their recent data Nigeria and South Africa is definitely going to be among the list.