Something "we" found when rehearsing Christmas carols is that some of the differences are in how people pronounce words or inflect their sentences - ; in many cases we would have to stop and decide WHICH way we as a group were going to sing a particular phrase -The way I sing "Mingulay" and the way a friend sing it have different inflections on several lines - and after several false starts we have learned to do it whichever way the one taking the lead does it - and neither is "wrong" as we have both heard recorded versions in both styles (and several others)
the same friend and I use VERY differnet phrasing on the round "Hey ho, nobody home" and the attempts to put the two together are hilarious, since we CAN adjust while singing in unison but tend to slip back into individual styles when we break into the round itself.