There are large number of sects among Muslims, even within traditionally famed Shia and Sunni. Classically, there are supposed to be 71 sects including those who believe in rebirth and idol worship as well. Prophet is supposed to have said that only one will go to Heaven. Further, due to bloodshed and rivalry among family members of Prophet and fraudulent killing of his grandsons, there have been historical reasons of sectarianism. Each sect has its own interpretation of Quran and Hadiths. And believe others to be false. No one understands original Quran. If one could understand, he would be prophet himself! So all go by translations and what Moulvis say. This has led to a blind cult mindset. So a Sunni follower will reject Shia sect. Wahabi will bomb Sufi. Sufi will talk of miracles that even Prophet could not do. It is a mess.
That is why smart Muslims like APJ Abdul Kalam and Pervez Hoodbhoy would follow their own intellect instead of following someone’s notion on a book no one understands.