That's actually peripheral to your point. If Islam were growing through conversions (of which I am skeptical), that further undermines your contention that people's religion is always determined by their origins.
The spread of Buddhism and Confuscianism also belies your point as well.
My point was that you'd be overwhelmingly likely to have your faith defined by geography. (which is undenuably true to all but the most willfully blind)
Not in any way did I suggest that it was a certainty.