There's a difference between what's written in holy books for any religion and how that religion plays out in practice. Tons of Catholic beliefs and practices are to be found nowhere in the Bible, for example. Ditto for both Sunni and Shia Islam and the Koran.

But when people leave a religion, they're not just leaving what the religion is on paper, because that's not what anyone engages with. They're leaving how the religion operates in practice, and that includes how people practice that religion day to day.