> Most groups look more homogeneous from the outside than the inside.

The converse is also true: Most groups look more heterogeneous from the inside than the outside. Again, to use the Christianity example: it takes a fair bit of inside baseball to figure out how various different sects and denominations are any different, but if you ask one of their priests, they'll be sure to tell you how Baptists and Episcopalians couldn't be more different.

It's a symptom of normalizing to an Overton window. From inside a church the belief that Jesus was just a guy who talked to God might seem an extreme fringe belief to someone who believes that Jesus is fully God, but from the outside we can lump them together as axiomatically holding that God Exists, because we have an Overton window ranging from "God doesn't exist" to "Allah talks to me everyday, and she gave me my promotion".