There's always different reasons for numbers. Some years ago JWs we're growing in Norway. Reason many JWs moved to Norway cause there was more job opportunities. Puerto Rico was other way around. Many moved to mainland and numbers of JWs was staggering. Either way you cut things. Only reason watchtower is not dropping massively in numbers are the strict DF rules. Remove DF arrangements and oh boy the drop they gonna have.
1. Other religious adherents move around too, this is hardly unique to JWs. (Hundreds of thousands of Polish Catholics moved to the UK in the past 20 years, for example)
2. A JW who moves to Norway, Puerto Rico etc., by definition has moved away from somewhere else. So while they are added to the Norway or Puerto Rico count, they are subtracted from the count in whatever country they came from.
Movement between countries, obviously, is no explanation for the growth in JW numbers worldwide, overall, unless we posit extra terrestrials.
Edit because I’ve run out of posts:
I don’t know any religious group that defines membership as attendance. Can you give an example? It’s not true of the groups I am familiar with. In fact many groups, such as Adventists and Catholics, maintain separate datasets for membership and attendance, precisely because they are not the same thing. Others don’t regularly publish attendance figures (though these may be gathered internally) such as the Church of Scotland which only publishes membership statistics not attendance numbers. When independent counts of attendance are conducted (see the work of Peter Brierley) they show that attendance at Church of Scotland churches is well under half the membership count.
I don’t know where you are coming from with this mistaken assumption that other groups count attendees as members.