Cultural Roots ~ Status of Religious Diversity in the U.S.
The study treats Christian denominations as members of the same religion, which if counted separately, would increase the ranking. But Islam, Hinduism, Buddhism and Judaism all have internal diversity, as well, and are considered as single religions in the study.
There’s an important distinction between religious diversity and religious freedom, which the report does not measure. The First Amendment to the U.S. Constitution guarantees the right to free exercise of religion, which is not always the case in other countries.