Is this a bit silly? At the end of the video, the three ladies are actually singing as a group rather, than as lead and two background singers. As a group, they are standing shoulder to shoulder. [[Nice to see them that way) Because of the choreography, Diana has to be about a foot in front of Mary and Cindy because if not, when all three of them extend their arms, they would be pushing each other out of the way. There are only two other options to allow all three to raise their arms at the conclusion. Diana could stand one foot back and the girls could cover Diana's face. Or they could split the group and have Cindy stand at stage right, Mary stand at stage left, and Diana stand at center stage. Then they could all raise their hands to their hearts content and no one's face would be covered.
Much of this also has to do with camera angles. There is a video of Jean, Mary and Lynda singing Bad Weather. When Jean takes her mike off the stand and moves from stage left position to center position, the camera angle has her blocking Lynda, almost as if she is standing in front of her. But she isn't. She is simply moving to the middle of the group, which is obvious as you continue to watch the video after camera angle is straight forward.
Conclusion: Much Ado About Nothing
https://www.youtube.com/watch?v=mzZXYmSop5Q