"I can't see any reason why Kulthean religions would be more homogeneous than our own, especially since the major gods seem not to care much about what the people may actually believe."
I can think of one big reason: the "Gods" of Shadow World are real, powerful entities that channel to their followers and actual manifest on the planet. That takes a bit of speculation and guess work out of the religions, unlike in our own real world? The Priests of such real beings would be incentivized to ensure that their followers were devout and following the correct entity. Once power structures: religions, temples and churches are created, then leadership will result. Even in our own world, with faith based religions, there is the Pope.
But hey, it's my take on things. Your mileage may vary.
Actually, I come from a different premise - the "Gods" are *not* really gods, and, unless they actually want it, do *not* channel to their followers constantly (all casters use their own ability to work magic, but some require a psychological symbol to 'make the magic work', and this symbol is the mental representation of a god - which explains why people can still work divine magic while not being faithful to the tenets of their 'inspiration', and also explain why people can worship, and 'gain' magic from, things that definitely are not gods.
The gods manifest, sure, but once in a couple of thousand years at best (especially the Lords of Orhan, who are notoriously loathe to intervene in the affairs of kultheans), which is quite often enough for the manifestation to become myth several times over, especially with the periodic upheavals that happen on Kulthea. And even when those manifestations are more frequent, divergences in cultures, antagonisms, difficulty to communicate, and simple physical barriers create variations that end up making different schism faiths and religions.
Kulthea, for me, is a fragmented world, and there's no reason why religion would not follow the trend, because is is first and foremost a cultural construct. After all, the Lords of Orhan do not *need* to be worshipped, and most of them do not even *want* it. They tolerate it, but it almost happens despite their best efforts. They accept it only because it serves the purpose of their opposition to the Charon gods and the Unlife, but they would rather leave the kulthean people to their own devices most of the time. And if they take a fancy to *some* people, they adapt their shape and tenets to the local culture, especially if the culture is somewhat isolated. The Gods of Charon are more 'involved', mostly because they gain pleasure from it, but even them do not need a homogeneous religion as long as the people do their bidding (basically, who care what those people do as long as it serves my whims ?)
So all in all, I find it much easier to believe in a world where religions are local and, even if there is some common grounds between religions that worship what is essentially the same God, if the people are different, their religion is different as well. So, when in Jaiman for instance, the faith of Phaon is different in the east (Tanara), the south (Rhakhaan) and the northwest (Saralis) - and very different in Remiriath because elves do not see the world in the same way as humans and their relation to the gods is different. Only faiths that distance themselves from temporal power can have any chance at homogeneity, because they don't crave temporal power and, as such, are not shacked to its mundane peculiarities.