I think it is certainly true that many immigrants are more vulnerable to conversion by JWs. Of course this only makes a difference to the countries they emigrate to in their end of year service reports. The overall worldwide report would still give the correct worldwide percentage increase figure. So yes it may be that many of the more developed countries would show less publishers if it were not for the immigrants that accept the JW religion.
Certainly over the years the UK have had more than their fair share of immigrants, so it would be interesting to hear from JWs if the 1% increase most years is made up of many immigrants joining the faith.