Namo Amida Butsu is a transliteration from Japanese. If you hear it being pronounced by a Japanese person you can hardly hear the last syllable ‘tsu’ at all. So what looks like two syllables sounds like one, pretty much. It follows that all of the Japanese nembutsu chants are six syllables, ‘Na’, ‘mo’, ‘A’, ‘mi’, ‘da’, and ‘butsu’ being a single syllable on the end.
For this reason it was decided that ‘Namo Amida Bu’ makes more sense in English, as we don’t know how to say ‘butsu’ which English speakers often read as two syllables.
This way we can use all of the Japanese six syllable chants easily.