Same source etc, but different religions ?

[Minimalist]

Market share.

A good explanation. I prefer to consider a more fundamental level, that markets (businesses, governments) and religions can be considered as self-replicating/perpetuating organisms in a not always friendly environment. They experience variation and selection pressures which result in their evolving to better fit the environs in which they find themselves. These somewhat regular though primarily chaotic environments lead different isolated populations of the replicators to evolve in different directions. Some sects go extinct, others flourish and spread. It's all (inorganic) biology(sic).

As usual, H. L. Mencken said it best:

Of learned men, the clergy show the lowest development of professional ethics. Any pastor is free to cadge customers from the divines of rival sects, and to denounce the divines themselves as theological quacks.