The group "Boney M" was originally just Frank Farian singing in a deep voice, backed up by himself singing, overdubbed, in a falsetto chorus. When one of his songs became a European hit, Farian hired performers to "front" the group on television: Jamaicans Liz Mitchell and Marcia Barnett, Maize Williams from Montserrat, and Bobby Farrell from Aruba. And, while the lineup changed through the years, it was well-known that Farian still sang all the male leads in the studio, and Maize Williams didn't sing on the records at all, "since her voice wasn't suited for this kind of music" (producer Farian's words).
So, only two members of the group sang on the records, although all four members sang on-stage, in concert. And, the band's concert sound was further enhanced by backing vocalists.
Numerous sources state that this was a common practice within the disco genre, and no one really cared in the 1970s...unlike when Farian did the same thing with Milli Vanilli in the 1980s.
Milli Vanilli quickly became one of the top pop acts of the late '80s, until it came out that Fab Morvan and Rob Pilatus did not sing the songs. Both were model/dancers that Frank Farian had found in a Munich, Germany dance club.