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.
And Australia is 163 billion!!!! Do you see the difference?
As I said before, it doesn't matter how many people gamble, what it matters is how much they can afford to gamble!
Disposable income in Nigeria is $700, it's $35000 in Australia!
How would people in Nigeria even be able to gamble as much when they don't even have that money?
You are right if we look at it from the angle of the cash inflow for these countries because US must definitely beat every other country as they have far more access to capital and various credit facilities. Furthermore, Per capital income of citizens of these countries are far higher than that of Nigerians and even if Nigeria have more gamblers by head count, they may not be able to measure with the countries listed . In other words, what an average American can bet with will likely be what the average Nigerian may be aiming to win. Therefore, I agree with the rating on the basis of volume.