Likewise me I was was even expecting Nigeria to be among the top three but not seeing them here means that we are still learning or the small gambling shops are not included as you clearly mentioned. I don't know how authentic the list is but seeing Australia in the list again makes me wonder, I never knew that the country also gamble that much.
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.