I'm flummoxed to remember the title or author of the book I read 5+ years ago which gave the results of a survey of self-identified religious affiliations in America. It presented US maps showing the distribution of each religion in America. Here in NY, you could easily get the impression that most Americans are Catholic, in Texas you might think they were mostly Baptist, and so forth.But the book also made the statement that immigrants to America were not representative of the populations of the countries they emmigrated from (that should be obvious, of course--otherwise whole countries would depopulate and immigrate here).
The book made the point that emmigrants from a non-christian country were disproportionately likely to be Christian--and emmigrants from a Christian country were disproportionately likely to be Protestant. Which only makes sense; the cultural resistance to leaving your homeland would be least among the groups least representative of the overall culture in any given country.
The effect is pretty strong, if emmigrants from a 90% muslim country are 77% non-muslim . . .