Some of it is interpretation.
The Catholics, Orthodox, and Protestants are all varieties of Christianity. They agree on many things, but disagree on a number of things. In a similar fashion, the Sunnis, Shiites, Wahabis, etc., agree on the broad matters, but have differences on certain points.
But they all agree on the need for the caliphate.
Sunni and Shia split is equivalent to the disagreement between Orthodox and Roman Catholic, but rather less than disagreement between Catholic and Presbyterian. Of course, Moslem view of that disagreement of what anyone else would see as inconsequential things is much more categoric. It is more Us vs Them and Them are Apostate and deserving of death.