It may have also been, at least in part, a simple matter of people changing their minds.
1972: No way, we're no using synths!
1973: No way we're no using synths (and we won't mention the stylophone on the inner sleeve).
1974-1976: No way, we're no using synths!
1977-1978: We're not using synths, yet we won't mention it in the credits.
1979: Hey, Rog's got an OB-X. Let's try it out!
1980-1995: Let's record an album with loads of synths!
1981-1988: Let's use drum-machines too!
1982: Let's tell John to fuck off while we play the bass-line on a JP8!
1982-1988: Let's leave the arpeggiator on way too long!
1984: Let's make a Xmas single without real drums!
And so on...
Same may have happened with lead vocals. Roger:
1972: This is MY song and I will sing it on the record.
1973-1976: This is MY song and I will sing it, full stop.
1977-1978: I wrote two songs this time, I don't mind if Freddie (co-)sings one.
1979: I wrote one - let's make a duet!
1980: Wrote two - one I'll sing, the other I'll give Fred the intro.
1981-1982: Let's duet on one and I'll let Fred sing the other.
1983: I've got a solo album and am working on another, so who cares if Fred sings my tracks?
1985: This is my song for the end credits. Fred's gotta do it, otherwise people won't know it's us!
1986: Developed a new idea from a former instrumental. Gotta give it to Fred, though. I'll sing in another band.
1987: Wrote a great song but once I heard Fred singing it I realised he could do it better.
1988: Wrote two songs for the album, and in both cases his voice suits them better. I'll sing lead in The Cross.
1989: Wrote two songs for the album, and in both cases his voice suits them better. I'll sing lead in The Cross.
Roger, AFAIK, didn't write anything for Queen (unless you count TCR but Fred had died by then so he couldn't sing that stuff anyway) after that so there you have it.
John hated HS. Frederick's favourite singer was not PR. Roger didn't compose 'Innuendo.' Witness testimonies are often inaccurate. Scotland's not in England. 'Bo Rhap' hasn't got 180 voices.