Easier to convert someone from Catholicism to Eastern Orthodoxy than it is to convert someone from Shia Islam to Norse Neopaganism after all.
Is it? Really? The game might think so, but I think that's rather flawed thinking. If it was so easy to convert orthodoxs to catholicism, don't you think the catholic church would send more preachers to Russia? After all, the catholic church LOVES converting as many people as it can. Are there really that many shiite muslims converting to sunni islam? Doubtful.
I'd be tempted to say that the closer the religions are, the harder it is to change from one to the other, as far as real life is concerned. Someone who has a spiritual crisis and who tells himself "Oh gosh, I've been wrong all of my life! The true nature of things is..." Will that sentence end with "something almost exactly to what I believed in but with a few minor cosmetic or administrative differences", or "something completely different then what I previously thought!"?
People undergoing serious enough spiritual questioning is not likely, in my mind, to hop on for a variant church. Not any more likely than a completely foreign one, at least. Part of this conversion is usually an act of social rebellion of sorts too, seeking a break with the surrounding social norms that aren't in harmony with the person's values.
The game probably thinks as you do, but I'd think it wrong. And prone to discourage religions to name anyone as being "variants", on the sole fear that it may help others convert their flocks.