Well, I think the stated goal was to have some people singing the melody too, so it would be melody+up a third +down a third throughout, rather than all open fifths around the melody. Trying to sing the whole thing as open fifths would have been fun (I love open fifths) but in a crowd like that somebody (like me) is going to find it feels right to sing the middle third.

("say your prayers! 'cause Santa Claus comes tonite.")