I had a hunch their numbers were either iflated or came from new immigrants.
Those not speaking the language, desperate to quickly try and learn it from somebody, or not knowing too many, if any, people in the new country THESE are people "ripe for the harvest" (JW Term) to convert. They know little if anything about this cult and will think they are truly loved, cared for, and have at last found the true organization Jehovah God is using.
Keep in mind many, many immigrants have heard of JW's if by name only and quite a few more are of some denomination of Christianity AKA Christendom. If one of the first people these people meet are JWs and they are the love-bombing kind, and not the cliquish kind, this may be very appealing to me.